Synthesis and Self-Assembly of Discrete Dimethylsiloxane-Lactic Acid Diblock Co-oligomers: The Dononacontamer and Its Shorter Homologues

Article abstract of DOI:10.1021/jacs.6b00629

Most of the theoretical and computational descriptions of the phase behavior of block copolymers describe the chain ensembles of perfect and uniform polymers. In contrast, experimental studies on block copolymers always employ materials with disperse molecular makeup. Although most polymers are so-called monodisperse, they still have a molecular weight dispersity. Here, we describe the synthesis and properties of a series of discrete length diblock co-oligomers, based on oligo-dimethylsiloxane (oDMS) and oligo-lactic acid (oLA), diblock co-oligomers with highly noncompatible blocks. By utilizing an iterative synthetic protocol, co-oligomers with molar masses up to 6901 Da, ultralow molar mass dispersities (D ≤ 1.00002), and unique control over the co-oligomer composition are synthesized and characterized. This specific block co-oligomer required the development of a new divergent strategy for the oDMS structures by which both bis- and monosubstituted oDMS derivatives up to 59 Si-atoms became available. The incompatibility of the two blocks makes the final coupling more demanding the longer the blocks become. These optimized synthetic procedures granted access to multigram quantities of most of the block co-oligomers, useful to study the lower limits of block copolymer phase segregation in detail. Cylindrical, gyroid, and lamellar nanostructures, as revealed by DSC, SAXS, and AFM, were generated. The small oligomeric size of the block co-oligomers resulted in exceptionally small feature sizes (down to 3.4 nm) and long-range organization.

Full text of DOI:10.1021/jacs.6b00629

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Article
Synthesis and Self-assembly of Discrete Dimethylsiloxane-Lactic Acid
Diblock Co-oligomers; the Dononacontamer and its Shorter Homologues
Bas Van Genabeek, Bas F.M. de Waal, Mark M.J. Gosens,
Louis M. Pitet, Anja R.A. Palmans, and E. W. Meijer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00629 • Publication Date (Web): 08 Mar 2016
Downloaded from http://pubs.acs.org on March 9, 2016
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Journal of the American Chemical Society
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Synthesis and Self-assembly of Discrete Dimethylsiloxane-
Lactic Acid Diblock Co-oligomers; the Dononacontamer and
its Shorter Homologues
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Bas van Genabeek, Bas F. M. de Waal, Mark M. J. Gosens, Louis M. Pitet, Anja R. A. Palmans, and
E. W. Meijer*
Institute for Complex Molecular Systems and Laboratory of Macromolecular and Organic Chemistry, Eindhoven University
of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands
Block copolymer, selfꢀassembly, monodisperse, oligomer, polydimethylsiloxane, polylactic acid, molar mass distribution,
precision synthesis.
ABSTRACT: Most of the theoretical and computational descriptions of the phase behaviour of block copolymers describe the
chain ensembles of perfect and uniform polymers. In contrast, experimental studies on block copolymers always employ materials
with disperse molecular makeup. Although most polymers are soꢀcalled monodisperse, they still have a molecular weight
dispersity. Here, we describe the synthesis and properties of a series of discrete length diblock coꢀoligomers, based on oligoꢀ
dimethylsiloxane (oDMS) and oligoꢀlactic acid (oLA); diblock coꢀoligomers with highly nonꢀcompatible blocks. By utilizing an
iterative synthetic protocol, coꢀoligomers with molar masses up to 6901 Da, ultraꢀlow molar mass dispersities (Ð ≤ 1.00002), and
unique control over the coꢀoligomer composition are synthesized and characterized. This specific block coꢀoligomer required the
development of a new divergent strategy for the oDMS structures by which both bisꢀ and monosubstituted oDMS derivatives up to
59 Siꢀatoms became available. The incompatibility of the two blocks makes the final coupling more demanding the longer the
blocks become. These optimized synthetic procedures granted access to multiꢀgram quantities of most of the block coꢀoligomers,
useful to study the lower limits of block copolymer phase segregation in detail. Cylindrical, gyroid, and lamellar nanostructures, as
revealed by DSC, SAXS and AFM, were generated. The small oligomeric size of the block coꢀoligomers resulted in exceptionally
small feature sizes (down to 3.4 nm) and longꢀrange organization.
contrary, N, fA and fB are variables established via synthetic
control.
■
INTRODUCTION
Several theoretical studies have emerged that enable
simulations to predict accurately the properties and selfꢀ
assembly behaviour of diblock copolymers.16–20 However,
relatively few models have accounted for chainꢀlength
dispersity, as result of the disproportionally longer calculation
times typically required for disperse systems.21–24 On the
contrary, almost every reported synthetic diblock copolymer,
although called monodisperse, exhibits molar mass dispersity
to a certain extent due to remaining challenges in preparing
wellꢀdefined polymers.25 This dispersity is a direct and
inevitable result of the living polymerization procedures
typically employed to produce diblock copolymers. Next to
dispersities in molar mass (hence in N), dispersities in the
composition (hence in fA versus fB) further disconnect theory
and experiments. In some cases, separation techniques, like
recycling GPC, were successful to minimize these dispersities
in diblock copolymers.26 Remarkably, to our knowledge, there
is a only limited number of examples of uniform synthetic
block copolymers with high χꢀparameter reported to date.27,28
Perfectly uniform polymers are ideal candidates to study the
chain length dependence of the polymer properties and to
substantiate results from simulations and the underlying
theoretical concepts.
Block copolymers (BCPs) are an intensively studied class of
materials exhibiting broad application window. The
a
propensity to selfꢀorganize into various morphological
structures having segregated domains on the length scale of
individualꢀchain dimensions makes such materials ideally
suited for various nanotechnologies. Lithographic resists for
nanoscale patterns,1–9 ultrafiltration membranes,10–12 stimuliꢀ
responsive materials,13 and nextꢀgeneration organic solar
cells14 highlight only few of these possible applications. Of the
many different BCPs synthesized and studied, diblock
copolymers are the ones that attracted most attention.
Diblock copolymers comprise two chemically
distinct polymeric segments, consisting generically of A and B
monomers, respectively, and linked together by a single
covalent connection. Three important physical parameters
dictate the selfꢀorganizing behaviour of diblock copolymers:
the FloryꢀHuggins interaction parameter, χ; the overall degree
of polymerization, N; and the polymer composition, expressed
as the volume fraction of block A (fA) or block B (fB).15 The
thermodynamic immiscibility of the blocks, reflected by χ, has
typical values between 0.02 and 0.4. The interaction parameter
is an intrinsic property of a given block combination. On the
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Page 2 of 11
Still, even the list of reported uniform, linear The synthesis of homochiral, discrete length
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homopolymers and oligomers with more than 20 repeating
units—excluding the biological structures made by solidꢀphase
synthesis—is remarkably short; covering (un)branched
oligomers of (S)ꢀlactic acid and εꢀcaprolactone has been
described by Hawker and coworkers,44,45 which we further
optimized for the fabrication of monodisperse oLA able to
couple to the oDMS block. In addition, we developed a novel
synthetic protocol to produce the discrete oDMS blocks, since
linear, discrete mass oligomers with more than eight repeating
units have not been reported. Literature examples that describe
the synthesis, modification or purification of oligomers based
on the dimethylsiloxane building block are scarce.53–55 Finally,
two different coupling strategies were optimized to combine
the discrete homoꢀoligomers to furnish a library of uniform
block coꢀoligomers, with the longest one having a total of 92
olefins,29–32
oligoethylene
glycols,33–39
polyethers,40,41
polyesters42–45 and conjugated oligomeric systems.46–50 Every
oligomer/polymer required its own synthetic approach,
typically divided into either a divergent or a convergent
approach, similar to the synthesis of dendrimers.51
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Scheme 1. Conventional synthesis of disperse PDMS-b-
PLA
siloxane and lactic acid repeating units: i.e.,
dononacontamer.
a
O
O
O
m
O
O
Si
Si
OH
O
Sn(Oct)2
Toluene
115 °C, 4 h
n
■
RESULTS AND DISCUSSION
Synthesis of discrete length oligodimethylsiloxane
O
O
O
Si
Si
blocks. The first route we developed to generate selectively
discrete length oligodimethylsiloxanes (Scheme 2) focused on
the formation of a bifunctional chlorosilane building block Cl-
Si4-H (3) with a length of four siloxane units. For
convenience, we will use the following abbreviations for the
oDMS blocks: A-Six-B, where A and B represent the oligomer
end groups and Six represents the number of siloxane repeating
units. Using building block 3, monodisperse oDMS blocks
were obtained by means of an iterative, twoꢀstep procedure.
First, the reaction of the chlorosilane with a monofunctional,
methyl endꢀcapped silanol (Me-Six-4-OH 5a-f) resulted in the
formation of a monofunctional silyl hydride (Me-Six-H 6a-f).
After purification, this silyl hydride was converted selectively
to the corresponding silanol by stirring in a phosphate
buffer/dioxane mixture in the presence of a Pd/C catalyst,
resulting in an increased chain length with four additional
siloxane repeating units realized in two steps.
O
OH
O
n
m
O
Figure 1: Graphical representation (calculated curve) of the molar
mass distribution of a BCP (Mn ≈ 6.9 kDa) with a dispersity of
1.05 (in black) and 1.000001 (in red). The yꢀaxis region between
3 and 90% is omitted for clarity.
Scheme 2. Linear route for siloxane monohydrides 6a-fa
O
a
Si
O
Si
O
+
Si
Si
Si
H
Here, we describe the synthesis and structural
analysis of a series of discrete diblock coꢀoligomer (BCO)
based on oligodimethylsiloxane (oDMS) and atactic
oligolactic acid (oLA). Previous reports showed that these two
constituents show an exceptionally high χꢀparameter,3 which
allows the formation of microphaseꢀseparated structures even
at low N.6,52 The synthesis of polydimethylsiloxaneꢀbꢀ
polylactide (PDMSꢀbꢀPLA) BCPs is conducted by stannous
Cl
H
Cl
O
Si
3
1
2
3
b
Si
Si
H
Si
Si
OH
O
O
2
2
4
5a
c
d
+
Si
Si
x-4
Si
Si
H
Si
Si
x-4
Si
Si
OH
Cl
OH
O
O
3
octoate catalysed ringꢀopening polymerization (ROP) of DL
ꢀ
O
H
O
x
5b-f
5a-f
3
6a-f
lactide (LA) with a hydroxylꢀterminated, lowꢀdispersity
PDMS macroꢀinitiator (see Scheme 1). Despite being robust
and fast, such polymerization techniques result in BCPs with
molar mass dispersities of at least Ð = 1.05. In Figure 1, a
calculated curve (Schulz–Zimm distribution) representing the
molar mass distribution for a polymer a dispersity Ð = 1.05 is
shown (black curve). As can be seen, this dispersity still
equals a relatively broad distribution of chain lengths. Thus, a
synthetic route that permits the synthesis of polymeric material
with dispersities approaching unity is desirable. Since the
oDMS and oLA blocks have such disparate chemical
compatibilities and functionalities, we chose to synthesize the
two discrete blocks separately.
a: x = 6
b: x = 10 e: x = 22
c: x = 14 f: x = 26
d: x = 18
a Reagents and conditions: (a) ACN, DMF (cat.), RT, 70 h (55%);
(b) Pd/C, dioxane, 1 M phosphate buffer (pH = 7), RT, 20 h
(91%); (c) pyridine, toluene, RT, 3 h (66ꢀ99%); (d) Pd/C, dioxane,
1 M phosphate buffer (pH = 7), RT, 20 h (89ꢀ94%).
The chlorosilane building block Cl-Si4-H (3) was
obtained in large quantities (>100 g per batch) by ring opening
of commercially available cyclotrisiloxane
1
with
chlorodimethylsilane 2. Separation of the desired material
from higher mass byproducts using vacuum distillation
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afforded the building block with four siloxane units in high disilanol 9h. Threefold repetition of these 2 steps eventually
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5
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7
8
purity and yield (65%). Next, commercially available, methyl
endcapped trisiloxane hydride 4 was converted into the
corresponding silanol 5a using the method described above.
Finally, reaction of 5a with chlorosilane 3 gave the elongated
hydride Me-Si7-H (6a). Repetition of these last 2 steps
resulted in the stepwise extension of the oDMS molecule with
4 siloxane units in every cycle, giving oligomers with 11, 15,
19, 23, and 27 siloxane units (6b-f), all in multiꢀgram
quantities, and overall yields of 50, 45, 38, and 27%,
respectively.
lead to the formation of more than 3 grams of disilanol
HO-Si40-OH (9k). The transformation of this symmetrical
siloxane into nonꢀsymmetrical silanol Me-Si55-OH (5l)
required
a
statistical reaction with siloxane chloride
Me-Si15-Cl (11). This material was obtained by the reaction of
Me-Si15-H (6c) with tetrachloromethane in the presence of
PdCl2 catalyst. Finally, condensation of the silanol 5l with
chlorosilane 3 resulted in 340 mg of discrete siloxane hydride
Me-Si59-H (6l). All siloxane hydrides were fully characterized
with 1HꢀNMR, 13CꢀNMR, size exclusion chromatography
(SEC) and MALDIꢀTOF analysis. No evidence was found for
the presence of oligomers with differing numbers of siloxane
repeating units.
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Scheme 3. Synthesis of longer siloxane monohydride 6la
Synthesis of discrete length oligolactic acid blocks.
For the synthesis of monodisperse, racemic lactic acid
oligomers having between 8 and 32 repeating lactic acid units,
we adopted a route first described by Hawker and coworkers
for the synthesis of homochiral L
ꢀlactic acid oligomers.45 In
this route, the use of orthogonal protective group chemistry
enabled sequential deprotection and coupling steps, resulting
in discrete oligolactic acid blocks. For convenience, we will
use the following abbreviations for the oLA blocks: C-LAy-D,
where C and D represent the oligomer end groups and LAy
represent the number of lactic acid repeating units. We
selected the same benzyl (Bn) ester and tertꢀbutyldimethylsilyl
(TBDMS) ether protective groups, owing to quantitative
deprotection under mild conditions. The benzyl protected
dimeric building block HO-LA2-Bn (13) was obtained in a
single step from the readily available DLꢀlactide 12 (i.e., a
mixture of Dꢀlactide and Lꢀlactide without the meso form) by
ring opening with benzyl alcohol in the presence of catalytic
amounts of camphorsulfonic acid (Scheme 4). Subsequently,
treatment of the alcohol with tertꢀbutyldimethylsilyl chloride
gave the double protected dimer TBDMS-LA2-Bn (14m) on
>30 g scale in high yield (90%). Monomeric byproducts
formed during the first reaction step were removed by column
chromatography. Subsequently, half of the material was
treated with Pd/C under a hydrogen atmosphere to remove the
benzyl ester and liberate the carboxylic acid 15m in
quantitative yield. Removal of the TBDMS protective group
was first attempted with tetrabutylammonium fluoride (TBAF)
solution in THF. However, the amphiphilic character of this
reagent and the significant water solubility of the hydroxy
dimer 16m lead to difficulties during the aqueous workup of
the reaction and a rather low yield of 55%. In addition, the
formation of substantial amounts of monomeric hydrolysis
products was observed. Therefore, BF3 etherate was used to
cleave the TBDMS ether without the formation of any
hydrolysis byꢀproducts.56 After purification by column
chromatography, hydroxy dimer 16m was obtained in 88%
yield. Formation of the doubly protected tetramer TBDMS-
LA4-Bn (14n) was conducted by ligation of the dimeric free
acid 15m and the hydroxy dimer 16m under standard
carbodiimide coupling conditions. After purification, the
tetramer was obtained in excellent yield (94%). By repeating
a Reagents and conditions: (a) NaHCO3, diethyl ether, water, 0–
20 °C, 4 h (99%); (b) pyridine, toluene, 0–20 °C, 3.5 h (81%); (c)
Pd/C, dioxane, 1 M phosphate buffer (pH = 7), 0–20 °C, 4.5 h
(94%); (d) pyridine, toluene, 0–20 °C, 3ꢀ5 h (47ꢀ94%); (e) Pd/C,
dioxane, 1 M phosphate buffer (pH = 7), 0–20 °C, 3ꢀ24 h (82ꢀ
100%); (f) PdCl2, CCl4, RT, 16 h (92%); (g) pyridine, toluene,
RT, 22 h (27%); (h) pyridine, toluene, RT, 3 h (65%).
The method described above is very effective for the
synthesis of relatively short siloxanes (less than 30 repeating
units). However, accessing longer oDMS blocks, containing
up to 59 repeating units, necessitated an alternative route using
symmetrical siloxane disilanols (9g-k). For this, dihydride
H-Si8-H (8) was prepared in two steps. Hydrolysis of building
block Cl-Si4-H (3) to HO-Si4-H (7) with NaHCO3 in a
water/diethyl ether mixture and subsequent coupling with
another equivalent of Cl-Si4-H (3) gave the desired material in
80% overall yield (Scheme 3). The disilane was converted into
the disilanol 9g in 94% yield by stirring in a phosphate
buffer/dioxane mixture in the presence of a Pd/C catalyst.
Next, an iterative, twoꢀstep procedure resulted in a chain
elongation by eight siloxane units in each consecutive cycle.
the
deprotection and coupling steps, combined with
purification by column chromatography after each reaction, a
range of hydroxyꢀterminated lactic acid oligomers 16o-u,
containing 8, 10, 12, 14, 16, 24 and 32 repeating units, were
obtained (22% overall yield for the longest oligomer). The
high purity and discrete length of these oligomers was
First, disilanol HO-Si8-OH (9g) was reacted with
2
equivalents of chlorosilane building block 3, resulting in
formation of H-Si16-H (10g). In a similar was as described
above, dihydride 10g was converted to the corresponding
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confirmed by 1HꢀNMR, 13CꢀNMR and MALDIꢀTOF analysis.
Page 4 of 11
BCOs [27-25] and [59-33] (Scheme 6). Acid functionalized
1
2
3
4
5
6
7
In all cases, only minor quantities (<0.5 mol%) of chains
containing ±1 repeating unit were observed.
oDMS 20f or l was transformed into the corresponding acid
chloride with Ghosez’s reagent.58 Reaction of the crude acid
chloride with hydroxyꢀterminated oLA 16t or u gave the
desired BCOs (Table 1, entries 11ꢀ12), albeit in low to
moderate yields of 48 and 18%, respectively, after
purification.
Scheme 4. The synthesis of lactic acid oligomers 16m-ua
8
9
Scheme 5. Synthesis of oDMS-oLA block co-oligomersa
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a Reagents and conditions: (a) BnOH, CSA, toluene, 80 °C, 8 h
(72%); (b) TBDMSCl, imidazole, DMF, RT, O/N (90%); (c) H2,
Pd/C, EtOAc, RT, 2 h (quant.); (d) BF3·Et2O, DCM, RT, 4 h (88ꢀ
96%); (e) EDC·HCl, DPTS, DCM, RT, 3 h (81ꢀ94%).
a Reagents and conditions: (a) allyl bromide, silver(I) oxide, Et2O,
reflux, O/N (60%); (b) Karstedt catalyst, toluene, 60 °C, 3h (43ꢀ
75%); (c) Pd/C, EtOAc, RT, 2 h (42ꢀ81%); (d) 15, EDC·HCl,
DPTS, DCM, RT, O/N (72ꢀ96%).
Synthesis of monodisperse BCOs. The coupling of
the wellꢀdefined oDMS and oLA blocks to yield the final
monodisperse block coꢀoligomers required functionalization
of the oDMS block with a carboxylic acid end group. As
shown in Scheme 5, the allyl ether of benzyl (S)ꢀlactate (18)
was obtained in moderate yield (60%) by a silver(I) oxide
mediated etherification of benzyl (S)ꢀlactate (17).
Subsequently, the lactate was coupled to the oDMS block
using the Karstedt hydrosilylation catalyst.57 Removal of the
benzyl ester by Pd/C catalysed hydrogenolysis, followed by
purification with column chromatography, afforded pure and
monodisperse acid functionalized oDMS Me-Six-LA1-COOH
(20c, e, f and l) having either 15, 23, 27 or 59 siloxane
repeating units, respectively. Subsequent carbodiimideꢀ
facilitated ligation of this block with hydroxyꢀterminated oLA
16 of various lengths, resulted in the formation of a series of
block coꢀoligomers [Si-LA] (e.g., [23-25] for the BCO
containing 23 siloxane and 25 lactic acid repeating units;
Table 1, entries 2ꢀ10). The combination of oDMS and oLA
lengths were chosen such that BCOs with values for fLA
between 0.25 and 0.50 were obtained, with which various
microstructures could be accessed. All block coꢀoligomers
were isolated in moderate to excellent yields and in very high
Scheme 6. Alternative coupling of the blocksa
a Reagents and conditions: (a) Ghosez’s reagent (1ꢀchloroꢀN,N,2ꢀ
trimethylꢀ1ꢀpropenylamine), DCM, RT, O/N (no purification); (b)
dry pyridine, DCM, RT, 5 h (18ꢀ48% (two steps)).
Molecular characterization of monodisperse
BCOs. Conventional size exclusion chromatography (SEC) is
insufficiently sensitive for determining accurate values of coꢀ
oligomer molar mass dispersity, since all BCOs had
dispersities that are far below the lower detection limit of this
technique (Ð ≈ 1.01). However, a striking difference was
observed between the discrete length BCO [15-17] and a
reference PDMSꢀPLA BCP [Ref] (Ð = 1.15) that was
synthesized in our group before6 (Figure 2 and Figures S16ꢀ
S18). Results that are more accurate were obtained by
MALDIꢀTOF analysis of the BCOs, showing that coꢀ
oligomers with the expected mass were present in each sample
(see Figure 3 and Figures S13ꢀS15). Only coꢀoligomers
[23-17], [23-25] and [59-33] contained traces of material that
1
purity, confirmed by HꢀNMR, 13CꢀNMR and MALDIꢀTOF
analysis (see Figures S1ꢀS15).
The reaction of the longest siloxane block
(Me-Si59-LA1-COOH 20l) with the longest oLA block
(HO-LA32-Bn 16u) under similar carbodiimide coupling
conditions did not give significant amounts of BCO. Instead,
most of the unreacted lactic acid block was recovered. In
addition, a siloxane derivative was isolated which most
probably is the Nꢀacylurea EDC adduct, a rearrangement
byproduct that is formed as result of the slow reaction of the
reactive Oꢀacylurea EDC adduct with the oLA block.
Therefore, an alternative strategy was employed to obtain
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differed from the correct coꢀoligomer length by one repeating 1). Furthermore, the Tg values found for oLA blocks in the
1
2
3
4
5
6
7
8
unit. For those BCOs, the molar mass dispersity was estimated
from the MALDIꢀTOF data by assuming that the relative peak
intensities corresponding to the desired and undesired
oligomer lengths directly represent the molar ratios of these
BCOs (see Table 1; entries 8,9 and 12). For the other coꢀ
oligomers, only an upper limit of for the dispersities could be
estimated, resulting in values (much) lower than 1.00001;
isotope distributions not taken into account.
BCOs are much lower than those found for the pure oLA
blocks (e.g., for HO-LA16-Bn (16s) a Tg of 19.1 °C was
found). In addition, all block coꢀoligomers, except the shortest,
[15-9], showed a second thermal transition at higher
temperatures (indicated with an asterisk in Figure 4). This
endothermic transition is attributed the orderꢀdisorder
transition (ODT) in which the phase segregated state goes into
the isotropic melt, a transition that is rarely observed in DSC
analyses of block copolymers.7,8,59,60 Smallꢀangle Xꢀray
scattering (SAXS) experiments (vide infra) confirm that the
samples are phase segregated and wellꢀorganized at
temperatures below the TODT. As expected, TODT increases in
the DSC experiments with increasing chain length for the
BCOs with similar fLA: [15-17], [23-25] and [27-25] (Table 1;
entries 3, 9 and 11, respectively). Interestingly, when using a
heating and cooling rate of 10 °C minꢀ1, sharp ODT signatures
were obtained for coꢀoligomers [15-17] and [23-25] (less than
1 °C broad). However, in the BCOs with a lower fLA, this
transition was broadened over a 5–6 °C temperature range.
The use of a temperature ramp of 5 °C minꢀ1 did not change
the results significantly. Currently, the origin of this behaviour
is under investigation.
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Figure 2: Normalized SEC traces (RI detector) for compounds
[15-17] and [Ref].
Figure 4: DSC traces (second heating run) of the BCOs
containing 23 siloxane units. The data is shifted for clarity.
Orderꢀdisorder transitions are indicated with an asterisk (*). A
heating/cooling rate of 10 °C minꢀ1 was used.
Determination of BCO microstructures. After
confirming the purity and thermal properties of the uniform
block coꢀoligomers synthesized, the microꢀphase separation
was analysed in bulk with smallꢀangle Xꢀray scattering and as
a thin film with atomic force microscopy.
Figure 3: MALDIꢀTOF spectra (DCTB matrix) for the BCO
series containing 23 siloxane units.
The monodisperse BCOs are chemically stable, even
after long storage in ambient conditions. MALDIꢀTOF
analysis of samples that were stored in air for 6 months at
room temperature did not show any degradation product.
Furthermore, the thermal stability of the BCOs was further
investigated with thermogravimetric analysis (TGA). The start
of the degradation onset at temperatures close to 300 °C
highlighted the remarkable stability for a polyester containing
material (Figure S19).
Bulk morphologies. Smallꢀangle Xꢀray scattering at
room temperature was used to determine the bulk morphology
and principal domain spacing (d*) for all BCO samples (Table
1). Azimuthal integration of the 2ꢀD transmission scattering
data resulted in 1ꢀD patterns. A selection of data collected at
20 °C is shown in Figure 5, representing 3 of the 4 classical
BCP morphologies and a disordered state. The scattering
profile of the short, compositionally asymmetric BCOs with a
low fLA value, [15-9] and [23-9] (Figure S22), showed only
low intensity and broad principal scattering peaks at 1.17 and
1.03 nmꢀ1, respectively. This suggests that these coꢀoligomers
are disordered, with only a weak composition inhomogeneity
present at room temperature. Furthermore, repeating the
measurement at ꢀ5 °C did not result in any additional ordering
for either sample. This is in accordance with the absence of an
ODT in the DSC trace of compound [15-9]. However, the
Differential scanning calorimetry (DSC) was used to
investigate the dependence of chain length on the thermal
transitions of the new block coꢀoligomers (see Figure 4 and
Figures S20ꢀS21). Although the glass transition temperature of
the oDMS block was too low (< ꢀ50 °C) to be observed with
our available DSC setꢀup, the glass transition of the oLA block
was evident, with values for Tg (oLA) between ꢀ32.7 °C and
+9.1 °C, monotonically increasing with increasing fLA (Table
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presence of an ODT signature for the [23-9] BCO at 20.7 °C
suggests that this coꢀoligomer should adopt a more ordered
state at ꢀ5 °C. At this moment we have no viable explanation
for this observation, although we presume that it is related to
kinetic effects.60
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Table 1: Molecular characterization data for monodisperse block co-oligomers.
1
2
[c]
[e]
[f]
Entry
Coꢀoligomer[a]
Mn
N[b]
fLA
Đ[d]
Tg
TODT
Phase[g]
d*[h]
[nm]
ꢀ
3
[Da]
[°C]
n.d.
[°C]
n.d.
4
5
6
7
8
9
1
2
2554[i]
33.5
25.6
32.6
35.0
36.8
38.5
40.3
42.0
49.0
45.0
53.7
98.5
0.46
0.34
0.48
0.25
0.29
0.32
0.35
0.37
0.46
0.31
0.42
0.30
1.15[j]
DIS
DIS
LAM[k]
[Ref]
1909
2486
2502
2646
2791
2935
3079
3655
3231
3952
6901
< 1.00001
< 1.00001
< 1.00001
< 1.00001
<1.00001
< 1.00001
1.00001
ꢀ32.7
0.5
ꢀ
ꢀ
[15-9]
3
72.9
20.7
42.2
52.3
70.8
82.5
148.9
91.3
151.3
n.d.
6.8
ꢀ
[15-17]
[23-9]
4
ꢀ32.0
ꢀ21.1
ꢀ17.2
ꢀ7.6
ꢀ6.8
9.1
DIS
5
CYL[k]
CYL
GYR
GYR
LAM
CYL
LAM
CYL
6.5
7.1
7.4
7.8
8.7
8.0
9.3
13.7
[23-11]
[23-13]
[23-15]
[23-17]
[23-25]
[27-15]
[27-25]
[59-33]
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23
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60
6
7
8
9
1.00001
10
11
12
< 1.00001
< 1.00001
1.00002
ꢀ6.6
5.8
n.d.
[a] Number of repeating siloxane and lactic acid units respectively; [b] Number of segments based on an 118 Å3
reference volume; [c] Lactic acid volume fraction, calculated using bulk densities for PDMS and PLA (0.95 g/ml and
1.24 g/ml, respectively); [d] Calculated from the relative peak intensities in the MALDIꢀTOF spectra; [e] Glass
transition of the oLA block; [f] Orderꢀdisorder temperature; [g] Bulk morphology determined with SAXS at room
temperature. DIS = disordered, CYL = cylindrical, GYR = gyroid, LAM = lamellar; [h] Domain spacing, calculated as
d* = 2π/q*; [i] Calculated from 1HꢀNMR integral ratios; [j] Determined by SEC; [k] After aging for 6 months at room
temperature; n.d. = not determined
Figure 5: SAXS data for the BCOs containing 23 siloxane units. The Figure 6: SAXS data for BCO [23-17]. Theoretical higher order
data is shifted vertically for clarity. Higher order Bragg reflections Bragg reflections for a gyroid morphology are indicated.
are indicated if present.
It is important to note here that the scattering profile
of the disperse polymer [Ref] (Đ = 1.15) lacks any evidence
for phase segregation or composition fluctuations, even though
this polymer has a comparable χN value. In contrast, the
scattering profile of the BCO [15-17] with similar lengths of
both blocks was dominated by a very narrow, high intensity
principal peak at q* = 0.93 nmꢀ1 (Figure S23). Still, the absence
of higher order reflections suggested that the presence of this
scattering peak merely is a result of very strong composition
fluctuations in the bulk material, lacking any longꢀrange
ordering. Interestingly, repetition of the scattering experiment
with a sample that was stored in a glass capillary at room
temperature for 6 months61 revealed additional reflections at
√4q* and √9q*, indicative for a lamellar phase. From the
position of the principal scattering peak, a principal interꢀ
planar (domain) spacing (d* = 2π/q*) of 6.8 nm was calculated.
For lamellae, this domain spacing is equal to the interꢀlamellar
distance (i.e., the sum of the lamellar thicknesses of both coꢀ
oligomer constituents), indicating that the lamellar thickness
of the individual blocks is approximately 3.4 nm. Similar
behaviour was observed for BCO [23-11] (Figure S24). First,
a singular, strong scattering peak was visible at q* = 0.96 nmꢀ1,
without additional Bragg reflections at higher qꢀvalues. After
aging for 6 months at room temperature,61 the formation of
hexagonally packed cylinders of the minor block (oLA) in a
matrix of the major block (oDMS) was observed, as is evident
from additional reflections at √3q*, √4q* and √7q*. From the
value for the domain spacing (d* = 6.5 nm), a cylinderꢀtoꢀ
cylinder distance (RIC = 2d*/√3) of 7.5 nm was extracted. The
diameter of the oLA cylinders (DCYL) was estimated to be 4.2
nm with the following relation (see Supporting Material for
the derivation of this formula):
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ꢌ
resolution (inset). Yet, the contrast (i.e., phase difference)
remains low (approximately 2 degrees). The low contrast
ꢉ
8ꢆ ꢈ∗
ꢉ
ꢃꢇ
1
2
3
4
5
6
7
8
ꢀꢁꢂꢃ ꢄ ꢅ
ꢋ
3 ∗ ꢊ
probably is a result of the slight intermixing of the oLA and
oDMS blocks, decreasing the difference in Young’s moduli
and thus phase difference between the two segregated blocks.
Moreover, the surface of the BCO film is anticipated to be
covered with a soft and sticky oDMS layer, due to the low
surface energy of this material with respect to that of oLA.63,68
This complicates the visualization of the underlying small
cylinders.63 Nevertheless, a cylinderꢀtoꢀcylinder distance of
9.1 nm could be extracted from the phase image by fast
Fourier transform (FFT) analysis, which agrees very well with
the value found with Xꢀray scattering (RIC = 9.2 nm).
√
To the best of our knowledge, the dimensions of the
lamellar and cylindrical structures formed by coꢀoligomers
[15-17] and [23-11], respectively, belong to the smallest
reported for a pristine diblock BCP system.2,4,7,8,60,62–64 This is
the more remarkable, since literature typically reports that
monodisperse polymers are less able to phaseꢀseparate at low
χN values.65ꢀ67 For BCO [23-13], with two additional lactic
acid repeating units, no prolonged aging times were required
to generate a microꢀphase separated system. The scattering
pattern with q* = 0.89 nmꢀ1 (d* = 7.1 nm) and additional
reflections at √4q* and √7q* suggest a cylindrical morphology
with RIC = 8.2 nm and DCYL ≈ 4.8 nm. Upon further increasing
fLA ([23-15] and [23-17]), characteristic peaks at for instance
√4/3q* and √7/3q* suggest the adoption of a gyroid
morphology. In case of the [23-17] BCO, even the 16th
(theoretical) reflection at √25/3q* could be observed (Figure
6), reflecting the high level of longꢀrange ordering in this
sample. A lamellar structure was observed for BCO [23-25],
with d* = 8.6 nm (q* = 0.73 nmꢀ1) and higher reflections at
√4q* and √9q*. The two 27ꢀsiloxane coꢀoligomers [27-15] and
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As expected, coꢀoligomer [59-33] formed a similar
type of microstructure, composed of an oDMS matrix with
oLA cylinders (RIC = 15.1 nm) parallel to the wafer surface
(Figure 7E). Due to the stronger segregation of this longer
BCO, a larger phase difference between the oLA and oDMS
blocks was present. Interestingly, less longꢀrange ordering of
the oLA cylinders was observed when compared to the
structure formed by BCO [27-15]. Probably, this decreased
order is a direct result of the higher stiffness and thus lower
chain mobility of the longer BCO. Indeed, after thermal
annealing of the sample for 2 hours in vacuum at 120 °C, a
system with better long range ordering was obtained (Figure
S28).
[27-25] showed cylindrical (d* = 8.0 nm, RIC = 9.2 nm, DCYL
=
5.4 nm) and lamellar (d* = 9.3 nm) morphologies, respectively,
in line with the expected structures based on the oLA volume
fractions (Figures S25ꢀS26). For a selection of polymers,
variable temperature SAXS experiments were performed. The
observed orderꢀdisorder transitions nicely support the
transitions observed in the DSC data (see for example Figure
S26 for BCO [27-25]). Finally, a cylindrical structure (d* =
13.7 nm, RIC = 15.8 nm, DCYL = 9.0 nm) was found for the
longest coꢀoligomer [59-33] (Figure S27).
Thinꢀfilm morphologies. Finally, we evaluated the
presence of ordered nanostructures in the coꢀoligomers
[27-15] and [59-33] in a thin film (< 20 nm) by atomic force
microscopy (AFM). A uniform, thin polymer film was
prepared by spinꢀcoating a 0.6 wt % solution of the BCO in
heptane onto a flat silicon substsrate with an AntiꢀReflective
topꢀCoating (ARC) of 93 nm thick. Important to note here is
that no annealing steps were applied after the preparation of
the thin layer. The height image of the spinꢀcoated compound
[27-15] revealed terrace formation (Figure 7A). Height
profiles, extracted from different regions of the image (see for
example Figure 7B), indicated that height fluctuations at each
terrace were small (< 1 nm). Moreover, a constant height
difference was found between two consecutive layers of 8.5±1
nm. This value is commensurate with the interꢀplanar spacing
that was extracted from the SAXS data for this BCO (d* = 8.0
nm). Based on these results and the tendency for this material
to form a cylindrical microstructure in bulk, we assume that
the film consists of a matrix of oDMS, in which oLA rich
cylinders are present as discrete layers, oriented parallel to the
wafer surface (Figure 7C). This is in accordance with previous
results obtained with a similar (disperse) PDMSꢀPLA system6
and confirmed by the presence of a fingerprintꢀlike line pattern
in the phase image at 500 x 500 nm resolution (Figure 7D).
Here, the alternating dark and light lines in this image
correspond to the cylinders of oLA and the oDMS matrix,
respectively. A clearer image was obtained at 150 x 150 nm
Figure 7: Tapping mode AFM images of BCOs [27-15] (AꢀD)
and [59-33] (E) as thin films on an ARCꢀmodified silicon wafer:
A) Height image with B) extracted height profile along the red
line and C) cartoon of the crossꢀsection perpendicular to the
wafer surface along the red line. Although the cartoon suggests
that the oDMS block wets the wafer surface, the exact
morphology of the wetting layer is unknown. D) Phase image of
BCO [27-15] monolayer, including an inset at higher
magnification. E) Phase image of BCO [59-33]. (light = oLA,
dark = oDMS).
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■
ASSOCIATED CONTENT
1
2
3
Experimental procedures and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
4
CONCLUSION
We have demonstrated that an iterative synthesis
5
6
7
8
■
AUTHOR INFORMATION
process with orthogonal protection/deprotection and coupling
strategies can be successfully employed to obtain discrete
length oligomers of dimethylsiloxane and lactic acid.
Subsequent ligation of these blocks afforded oDMSꢀoLA
diblock coꢀoligomers of molecular weights up to 6.9 kDa (DP
= 92) and various oLA volume fractions. As a result of the
mild and practically neutral conditions that were used during
the carbodiimide facilitated ligation and acid chloride
formation, practically no degradation of the oligomers took
place, resulting in extremely low molar mass dispersities (Ð ≤
1.00002). In principle, this synthesis strategy could be
extended to generate even longer BCOs, BCOs composed of a
different type of polyester block or BCOs containing moieties
that provide additional stabilizing interactions. However, when
interested in multigram quantities, the synthesis of the
dononacontamer is at present time the limit from the siloxane
point of view. Additionally, there is a practical length limit for
the individual blocks around 100ꢀ150 repeating units due to
decreasing reaction rates as a result of endꢀgroup dilution.
Moreover, ligation of two highly incompatible blocks becomes
extremely inefficient at individual oligomer lengths of 50
repeating units and higher. However, much to our surprise, the
relatively short BCOs possess already sharp orderꢀdisorder
transitions of phaseꢀseparated morphologies. Although,
literature data suggests that lowering the dispersity of block
Corresponding Author
* e.w.meijer@tue.nl
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■
ACKNOWLEDGMENT
This work is financed by the Dutch Ministry of Education,
Culture and Science (Gravity program 024.001.035), the Royal
Netherlands Academy of Arts and Sciences, and the European
Research Council (FP7/2007–2013, ERC Advanced Grant No.
246829). We thank Timo Sciarone for the TGA measurements
and we gratefully thank Prof. Craig J. Hawker for discussions and
support in the development of the original oLA synthesis route.
■
ABBREVIATIONS
TBDMS, tertꢀbutyldimethylsilyl; Bn, benzyl; EDC·HCl, 1ꢀethylꢀ
3ꢀ(3ꢀdimethylaminopropyl)carbodiimide hydrochloride; DPTS,
N,Nꢀdimethylaminopyridinium pꢀtoluene sulfonate; CSA,
camphorsulfonic acid; DCTB, transꢀ2ꢀ[3ꢀ(4ꢀtertꢀbutylphenyl)ꢀ2ꢀ
methylꢀ2ꢀpropenylidene]malonoꢀnitrile.
■
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