Temperature induced phase separation of luminescent silica nanoparticles in Triton X-100 solutions

The aggregation and cloud point behavior of Tb(III)-doped silica nanoparticles has been studied in Triton X-100 (TX-100) solutions at various concentration conditions by fluorimetry, dynamic light scattering, electrophoresis and transmission electron microscopy methods. The temperature responsive behavior of nanoparticles is observed at definite concentration of TX-100, where the aggregation of TX-100 at the silica/water interface is evident from the increased size of the silica nanoparticles. The reversible dehydration of TX-100 aggregates at the silica/water interface should be assumed as the main reason of the temperature induced phase separation of silica nanoparticles. The distribution of nanoparticles between aqueous and surfactant rich phases at the phase separation conditions can be modified by the effect of additives.     

Aggregation Behavior of Polyether Block Copolymers with Dendritic Structure in Aqueous Solutions

Two dendritic block copolymers with the same contents of PPO and PEO but different branches and blocks were synthesized by anion polymerization. Their aggregation behavior and aggregation morphology were investigated by steady-state fluorescence and transmission electron microscopy. For SD64 copolymer with PPO-PEO diblock branch, it can be proved that only intermolecular aggregates are formed, the aggregation number and aggregation diameter are increased with concentration. Whereas for SD343 dendritic copolymer with PPO-PEO-PPO triblock branch, hydrophobic PPO chains located on the edge of SD343 copolymers can associate within the same polymer chain and also between different polymer chains, so the aggregates were inclined to change from intramolecular micelles to intermolecular clusters with concentration increasing.     

Physical gels based on charge-driven bridging of nanoparticles by triblock copolymers

We have prepared an aqueous physical gel consisting of negatively charged silica nanoparticles bridged by ABA triblock copolymers, in which the A blocks are positively charged and the B block is neutral and water-soluble. Irreversible aggregation of the silica nanoparticles was prevented by precoating them with a neutral hydrophilic polymer. Both the elastic plateau modulus and the relaxation time increase slowly as the gel ages, indicating an increase both in the number of active bridges and in the strength with which the end blocks are adsorbed. The rate of this aging process can be increased significantly by applying a small shear stress to the sample. Our results indicate that charge-driven bridging of nanoparticles by triblock copolymers is a promising strategy for thickening of aqueous particle containing materials, such as water-based coatings.     

Interactions Between Polymers and Nanoparticles: Formation of “Supermicellar” Hybrid Aggregates

Abstract

When polyelectrolyte‐neutral block copolymers are mixed in solutions to oppositely charged species (e.g., surfactant micelles, macromolecules, proteins, etc.), there is the formation of stable “supermicellar” aggregates combining both components. The resulting colloidal complexes exhibit a core‐shell structure, and the mechanism yielding to their formation is electrostatic self‐assembly. In this contribution, we report on the structural properties of “supermicellar” aggregates made from yttrium‐based inorganic nanoparticles (radius 2 nm) and polyelectrolyte‐neutral block copolymers in aqueous solutions. The yttrium hydroxyacetate particles were chosen as a model system for inorganic colloids, and also for their use in industrial applications as precursors for ceramic and opto‐electronic materials. The copolymers placed under scrutiny are the water‐soluble and asymmetric poly(sodium acrylate)‐b‐poly(acrylamide) diblocks. Using static and dynamical light‐scattering experiments, we demonstrate the analogy between surfactant micelles and nanoparticles in the complexation phenomenon with oppositely charged polymers. We also determine the sizes and the aggregation numbers of the hybrid organic–inorganic complexes. Several additional properties are discussed, such as the remarkable stability of the hybrid aggregates and the dependence of their sizes on the mixing conditions.     

Reversible controlled assembly of thermosensitive polymer-coated gold nanoparticles

Aggregation of thermosensitive polymer-coated gold nanoparticles was performed in aqueous solution in the presence of a triblock copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic P123, PEO(20)-PPO(68)-PEO(20)). The gold nanoparticles, AuNPs, which are covered by thermosensitive statistical copolymers poly(EO(x)-st-PO(y)), aggregate when the temperature is higher than the phase transition temperature of the polymer, leading to a macroscopic precipitation. The presence of Pluronic chains in solution prevents the uncontrolled aggregation of the AuNPs at higher temperature than both the aggregation temperature of the AuNPs (T(agg)) and the critical micellization temperature (cmt) of the Pluronic. The size, the colloidal stability, and the optical properties of the AuNPs aggregates are modulated as a function of the P123-to-AuNP ratio, which constitutes the critical parameter of the system. Moreover, the AuNP aggregation is totally reversible upon decreasing the temperature below T(agg). Our approach constitutes an easy way to the formation of well-controlled nanoparticle aggregates with well-defined sizes. The resulting aggregates have been characterized by UV-vis spectroscopy, dynamic light scattering, and electron microscopy.     

PLA-b-PEO-b-PLA三嵌段共聚物分子聚集体性质的研究

采用开环聚合的方法 ,合成了组成不同的PLA b PEO b PLA三嵌段共聚物 .滴加选择性溶剂水于共聚物的良溶剂溶液中 ,制备了共聚物以水为介质的“平头”聚集体胶束溶液 .把聚集体胶束溶液浇铸在云母片上 ,采用扫描探针显微镜 (SPM)表征了其形貌和表面微粘弹性 .发现脱离了极性介质水的聚集体的表面性质发生了不均一化 ,聚集体的顶部比相连接的部分具有较高的储能模量 .聚集体环境的改变使聚集体中不同嵌段的迁移导致了这种表面粘弹性的不均一 .另外 ,采用动态光散射的方法测量了体系溶液中聚集体胶束的尺寸 .实验发现光散射所得到的聚集体的尺寸远远大于SPM所得到尺寸 .增加聚合物的起始浓度使聚集体胶束的尺寸以及多分散性都在不同程度上增大 .然而聚合物的不同 ,这种增加的程度会有比较大的差别     

Supramolecular formation of triblock copolymers in polar/nonpolar solvents

Triblock copolymers could form supramolecules in either polar or nonpolar solvents at appropriate concentration and temperature ranges or in the presence of additives. The association properties and the structure of supramolecules of PEO-PPO-PEO and PPO-PEO-PPO (PEO and PPO refer to poly(oxyethylene) and poly(oxypropylene), respectively) triblock copolymers in xylene and/or water were investigated by using light scattering, small-angle neutron scattering, and small-angle X-ray scattering. The association process of aqueous solution or water-rich ternary systems was entropy driven and temperature played an important role. The additive, e.g., water in the oil-rich ternary system, played a very important role on the micellization of PEO-PPO-PEO, e.g., Pluronic L64, in xylene. The micelles had a core-shell structure and the micellar shell was rather heavily solvated. At high copolymer concentrations, large aggregates with a lamellar structure was formed and the amount of large aggregates increased with increasing copolymer concentration before gel formation.     

Poly(chloropropylmethyl-dimethylsiloxane)–polyurethane elastomers: Synthesis and properties of segmented copolymers and related zwitterionomers

A series of polyurethane block copolymers based on hydroxybutyl terminated poly(chloropropylmethyl-dimethylsiloxane) and poly(tetramethylene oxide) soft segments of molecular weights 2100 and 2000, respectively, were synthesized. The hard segments consisted of 4,4′-methylenediphenylene diisocyanate (MDI) that was chain extended with either 1,4-butanediol (BD) or N-methyldiethanolamine (MDEA). The materials chain extended with MDEA were ionized using 1,3-propane sultone. The weight fraction of the hard segments was in the range 0.30–0.45. The effect of mixed soft segments, chain extenders, and zwitterionization on the extent of phase separation and physical properties was studied by utilizing differential scanning calorimetry and dynamic mechanical, stress-strain, and Fourier Transform Infrared spectroscopy experiments. All of these short segment block copolymers showed nearly complete phase separation. The zwitterionomer materials exhibited ionic aggregation within the hard domains. Although hard segment crystallinity or ionic aggregation did not affect the morphology, hard domain cohesion was important in determining the tensile and viscoelastic properties of these elastomers.     

聚醚树枝体线性聚(N-异丙基丙烯酰胺)不对称嵌段共聚物的 合成及自组装

采用原子转移聚合方法合成了聚N-异丙基丙烯酰胺和聚醚树技体的不对嵌段共聚物Dendr.PE-PNI-PAM。实验结果表明Dendr.PE-PNIPAM分子在水中能通过疏水缔合作用形成具有双分子膜结构的超分子聚集体。临界缔合浓度(CAC)、聚集体的大小及形貌对树枝体的代数具有明显的依赖关系。该聚集体对温度刺激具有响应性,并在人体体温温度(37.5℃)发生相转变。在高于相转变温度时,Dendr.PE-PNIPAM分子形成管状、带状等多重形态的超级结构的聚集体。     

Aggregation of poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solution: DPD simulation study

The dissipative particle dynamics (DPD) simulation method was applied to simulate the aggregation behavior of three block copolymers, (EO)16(PO)18, (EO)8(PO)18(EO)8, and (PO)9(EO)16(PO)9, in aqueous solutions. The results showed that the size of the micelle increased with increasing concentration. The diblock copolymer (EO)16(PO)18 would form an intercluster micelle at a certain concentration range, besides the traditional aggregates (spherical micelle, cylindrical micelle, and lamellar phase); while the triblock copolymer (EO)8(PO)18(EO)8 would form a spherical micelle, cylindrical micelle, and lamellar phase with increasing concentration, and (PO)9(EO)16(PO)9 would form intercluster aggregates, as well as a spherical micelle and gel. New mechanisms were given to explain the two kinds of intercluster micelle formed by the different copolymers. It is deduced from the end-to-end distance that the morphologies of the diblock copolymer and triblock copolymer with hydrophilic ends were more extendible than the triblock copolymer with hydrophobic ends.     

Design, synthesis, and aqueous aggregation behavior of nonionic single and multiple thermoresponsive polymers

Nonionic water-soluble poly(acrylamide)s and poly(acrylate)s were synthesized by RAFT and ATRP methods. Similar to the synthesized poly(N-isopropylacrylamide) and poly(N-acryloylpyrrolidine), aqueous solutions of statistical acrylate copolymers bearing two different oligo(ethylene oxide) side chains showed a sharp clouding transition upon heating beyond characteristic temperatures. The temperature of the cloud point can be easily fine tuned by the copolymer composition. As for poly(N-isopropylacrylamide) and poly(N-acryloylpyrrolidine), the cloud-point temperatures of these statistical copolymers are rather insensitive to changes in the molar mass or the NaCl content of the solutions. Also, ternary triblock copolymers containing one permanently hydrophilic block and two different thermoresponsive blocks were synthesized, varying the block sequence systematically. Their aggregation in aqueous solution was followed by turbidimetry and dynamic light scattering. Depending on the heating process and the triblock sequence, micellar aggregates of 40 to 600 nm size were found. The thermally induced aggregation behavior depends sensitively on the block sequence but is also subject to major kinetic effects. For certain block sequences, a thermally induced two-step association is observed when heating beyond the first and second cloud points of the thermoresponsive blocks. However, the thermal-transition temperatures of the block polymers can differ from the thermal-transition temperatures of the individual homopolymers. This may be caused by end-group effects but also by mutual interactions of the different blocks in solution, as physical mixtures of the homopolymers exhibit deviations from a purely additive thermal behavior.     

Block copolymers in the synthesis of gold nanoparticles. Two new approaches: Copolymer aggregates as reductants and stabilizers and simultaneous formation of copolymer aggregates and gold nanoparticles

In this article, innovative applications of amphiphilic triblock and pentablock copolymers in the synthesis of gold nanoparticles are reported. The synthesis of gold nanoparticles is performed using two methods. In the first method, micellar aggregates of block copolymers and AuCl₄^? ions directly react in water; the nanoparticles obtained by this method are variable in size and are associated with copolymer aggregates. In the second method, two processes take place simultaneously: the aggregation of block copolymers and the reduction of Au (III) by the copolymers to form nanoparticles. In contrast with the first method, in this case, the nanoparticles obtained are located inside the copolymer aggregates. In both methods of synthesis, the block copolymers act simultaneously as reducing and stabilizing agents. To understand the role of copolymer aggregates in the synthesis of nanoparticles, molecular simulation methods are used. The gold nanoparticles, copolymer aggregates, and nanocomposite systems are characterized using transmission electron microscopy and dynamic light scattering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3069–3079     

Thermogelling of double hydrophilic multiblock and triblock copolymers of N,N-dimethylacrylamide and N-isopropylacrylamide: chain architectural and Hofmeister effects

A series of thermoresponsive double hydrophilic (AB)(n) multiblock and ABA triblock copolymers of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) with varying sequence lengths were synthesized via successive reversible addition-fragmentation chain transfer (RAFT) polymerizations by employing polytrithiocarbonate as the chain transfer agent. Previously, we reported that multiblock copolymers in dilute aqueous solutions can form either unimolecular or multimolecular micelles at elevated temperatures depending on the relative chain lengths of PDMA and PNIPAM sequences (Zhou et al. Langmuir 2007, 23, 13076-13084). In this follow-up work, we further explored and compared the chain architectural (multiblock vs triblock) and Hofmeister effects (addition of various sodium salts) on the gelation behavior of multiblock and ABA triblock copolymers at high concentrations and attempted to establish a correlation between the aggregation behavior and gelation properties of multiblock copolymers at low and high polymer concentrations, respectively. It was found that only m-PDMA(p)-PNIPAM(q) multiblock copolymers with PDMA and PNIPAM sequence lengths located within a specific range can form physical gels at elevated temperatures. Rheology measurements revealed that multiblock copolymers possess considerably lower critical gelation temperatures (CGT) and higher gel storage modulus, G'(gel), as compared to those of PNIPAM-b-PDMA-b-PNIPAM triblock copolymers possessing comparable sequence lengths. The addition of inorganic sodium salts can effectively facilitate thermogelling for multiblock and triblock copolymers, resulting in decreasing CGTs and critical gelation concentrations (CGCs) in the order of Hofmeister series with increasing hydration capabilities. The unique thermogelling behavior of aqueous multiblock copolymer solutions in the absence and presence of inorganic salts, as compared to that of ABA triblock copolymers, augurs well for their potential applications in various fields such as biomaterials and biomedicines.     

Synthesis of polyrotaxanes consisting of multiple α-cyclodextrin rings threaded on reverse Pluronic PPO-PEO-PPO triblock copolymers based on block-selected inclusion complexation

New polyrotaxanes, in which multiple α-cyclodextrin (α-CD) rings were threaded and imprisoned on a reverse Pluronic PPO-PEO-PPO triblock copolymer chain and capped by two 2,4-dinitrophenyl groups at the two ends, were synthesized based on the block-selected inclusion complexation between the PPO-PEO-PPO triblock copolymers and α-CD. The polyrotaxanes were isolated and purified through size exclusion chromatography, and the chromatograms showed that the polyrotaxanes had high molecular masses as compared with their respective components. The polyrotaxanes were further characterized by using ¹³C and ¹H 1D and 2D NMR spectroscopy, wide-angle X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. It was found that the threading PPO-PEO-PPO triblock copolymers became more thermally stable because they were complexed by the imprisoned α-CD. The studies demonstrated that the numbers of α-CD rings imprisoned in the polyrotaxanes were directly related to the PEO block length, rather than to the PPO block length. In all the three polyrotaxanes synthesized in this work, the central PEO block was fully covered and complexed by α-CD stoichiometrically, while the flanking PPO bocks remained free and uncomplexed. Therefore, the new polyrotaxanes took a PPO-polyrotaxane-PPO triblock architecture. The polyrotaxanes with such block architecture may be interesting supramolecular precursor for designing novel functional materials.     

Temperature-induced self-assembly of triple-responsive triblock copolymers in aqueous solutions

A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was synthesized by sequential reversible addition-fragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20-25 °C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40-55 and 70-80 °C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 °C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.     

Thermoresponsive hydrogels from symmetrical triblock copolymers poly(styrene-block-(methoxy diethylene glycol acrylate)-block-styrene)

A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20-40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel-sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP(n) about 450), gels had already formed at 3.5 wt % at low temperatures. The gel-sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.     

Light and neutron scattering study of PEG-oleate and its use in emulsion polymerization

Steric stabilization of colloids forms a robust mechanism to obtain colloids that are stable in a variety of environments, and that can be used to study the phase behavior of hard or soft spheres. We report the synthesis of sterically stabilized colloids in an aqueous environment using readily dissolvable surfactants, with an unsaturated hydrophobic tail. We synthesized a new surfactant by esterification of a poly(ethylene glycol) (PEG) chain of 4.1 kg/mol with oleic acid, called PEG4OA. The micellization of PEG4OA was characterized by light and neutron scattering, which yielded values for the aggregation number and the overall size that are in excellent agreement with a comparable surfactant with a saturated octadecane chain, Brij 700. We successfully used PEG4OA in the emulsion polymerization of polystyrene colloids. In comparison with the smaller surfactant Tween 80, PEG4OA yielded smaller colloids with radii around 50 nm, and the addition of 1-dodecanethiol reduced the formation of aggregates during the synthesis. A contrast variation study with small angle neutron scattering (SANS) showed that a dense PEG layer was grafted to the colloid surface.     

Stimuli‐responsive ABC triblock copolymers by sequential living cationic copolymerization: Multistage self‐assemblies through micellization to open association

Stimuli‐responsive ABC triblock copolymers with three segments with different phase‐separation temperatures were synthesized via sequential living cationic copolymerization. The triblock copolymers exhibited sensitive thermally induced physical gelation (open association) through the formation of micelles. For example, an aqueous solution of EOVE₂₀₀‐b‐MOVE₂₀₀‐b‐EOEOVE₂₀₀ [where EOVE is 2‐ethoxyethyl vinyl ether, MOVE is 2‐methoxethyl vinyl ether and EOEOVE is 2‐(2‐ethoxy)ethoxyethyl vinyl ether; the order of the phase‐separation temperatures was poly(EOVE) (20 °C) < poly(EOEOVE) (41 °C) < poly(MOVE) (70 °C)] underwent multiple reversible transitions from sol (<20 °C) to micellization (20–41 °C) to physical gelation (physical crosslinking, 41–64 °C) and, finally, to precipitation (>64 °C). At 41–64 °C, the physical gel became stiffer than similar diblock or ABA triblock copolymers of the same molecular weight. Furthermore, the ABC triblock copolymers exhibited Weissenberg effects in semidilute aqueous solutions. In sharp contrast, another ABC triblock copolymer with a different arrangement, EOVE₂₀₀‐b‐EOEOVE₂₀₀‐b‐MOVE₂₀₀, scarcely exhibited any increase in viscosity above 41 °C. The temperatures of micelle formation and physical gelation corresponded to the phase‐separation temperatures of the segment types in the ABC triblock copolymer. No second‐stage association was observed for AB and ABA block copolymers with the same thermosensitive segments found in their ABC counterparts. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2601–2611, 2004     

Thermodynamic‐Dependent Size‐Selection of ZnSe Nanoparticles in Amphiphilic Triblock Copolymer Systems

ZnSe colloidal nanoparticles prepared by the air‐insensitive starting reagents, zinc chloride and selenium powder, have been size‐selected in the Pluronic amphiphilic triblock copolymer [(EO)_x(PO)_y(EO)_x] systems. The size‐selection mechanism between the ZnSe nanoparticles and the triblock copolymers systems is a thermodynamic‐dependent effect. We observe that nanoparticles with special volume (Vs) are trapped first by the triblock copolymers due to the faster entropic depletion interaction arising from the addition of surfactant‐template (micelles) to colloidal nanoparticles. On the other hand, nanoparticles with sizes larger or smaller than Vs will not interact with the surfactant‐templates. They either precipitate quickly by gravity (larger than Vs) or still retain their thermal motion in the aqueous phase (smaller than Vs) when Vs nanoparticles are caught by the surfactant‐templates.     

Synthesis and characterization of (AB)n-type poly(l-lactide)-poly(dimethyl siloxane) multiblock copolymer and the effect of its macrodiol composition on urethane formation

An (AB)_n-type multiblock copolymer containing alternating poly(l-lactide) (PLLA) and poly(dimethyl siloxane) (PDMS) segments was synthesized by chain extension of hydroxyltelechelic PLLA-PDMS-PLLA triblock copolymers, which were prepared by the ring-opening polymerization of l-lactide initiated by α,ω-functionalized hydroxyl poly(dimethyl siloxane), using 1,6-hexamethylene diisocyanate as a chain extender. The triblock and the multiblock copolymers were characterized by FT-IR, ¹H NMR and GPC. From the results of thermal analysis, two glass transition temperatures which were measured by DSC showed the occurrence of phase separation phenomena in the triblock and multiblock copolymers because of the difference of solubility parameters between PLLA and PDMS segments. The effect of the chemical composition of the triblock copolymers, including the Mw and the constitutive segment chain length of the macrodiol, on the development of the Mw of the multiblock was discussed based on diffusion effect. Furthermore, the consumption of the isocyanate groups was determined by FT-IR to investigate the dependence of the reaction kinetics of the urethane formation on the chemical composition of the triblock copolymer. The results reveal that the order of the chain extension reaction depended on the Mw of the triblock copolymer: a second order reaction was transformed into a third reaction as the Mw of the triblock copolymer increased from 7000 to 25,000 (g/mol) perhaps because of the inhibition of the formation of an active complex involved in the catalyzed-urethane reaction by the polymer chain aggregation. Finally, the mechanical properties of the multiblock copolymers demonstrated that the introduction of the extremely flexible PDMS segment substantially improved the elongation at breakage, and the tensile strength and the tensile modulus declined due to the intrinsic elasticity of such segments.