Dynamical mechanical behavior of copolymers made of styrene and methyl methacrylate: Random, alternate and diblock copolymers

The dynamics of copolymers made of styrene and methyl methacrylate and of different architecture (three diblock, two random and one alternate) have been studied using a dynamic mechanical spectroscopy technique (DMTA). The scanning calorimetric results indicate that there is only one glass transition except for two of the diblock copolymers. In addition, only one broad mechanical relaxation is observed in all the copolymers studied in this work. However, it has not been possible to build master curves for the complex Young modulus E* for the copolymers. In fact the relaxation spectra calculated from E* are bimodal. In order to describe the relaxation functions of the samples, it has been necessary to use two Kohlraush-Williams-Watts functions at each of the temperatures studied. The relaxation times of the two dynamic contributions can be described by Arrhenius laws, which is probably due to the relatively narrow temperature range for which the relaxation can be studied within the frequency range experimentally accessible. The stretching parameters increase linearly with T, which indicates that both dynamic transitions broaden as T is decreased.     

Orientational ordering in blends of flexible and rigid diblock copolymers

A self-consistent mean-field computer simulation of ordering in blends of flexible and rigid rodlike diblock copolymers is performed. Each type of rigid block is selective toward one of the types of flexible blocks and is incompatible with the other. Flexible diblocks form a predominant component of the blend with parameters providing formation of a stable hexagonal morphology. As a result, orientational ordering is induced in the blend by a minor admixture of rigid diblocks, while the type of ordering is determined by the composition of the rigid diblocks. Three different types of orientational ordering, which having a clearly pronounced electromagnetic analogy, are established during variation in the composition of the rigid diblocks. Near a cylindrical micelle of the hexagonal morphology, the vector fields of primary directions of the orientational-order-parameter tensors for the three orientation types are highly similar to the magnetic field of the infinitely long rectilinear direct current, the cylindrical solenoid magnetic field with a constant linear current density, and the electrostatic field of a uniformly charged straight line, respectively.     

Molecular simulation of diblock copolymers; morphology and mechanical properties

Molecular modeling and computer simulations were used to construct, visualize, control and predict nanostructures with specific morphologies, self-assembling regions and mechanical properties associated to poly(styrene)-poly(isoprene) and poly(styrene)-poly(methyl methacrylate) diblock copolymers. Molecular structures of each diblock copolymer were constructed and used to obtain a Gaussian chain constituted of beads. Segment-segment interactions representing the chemical nature of the systems were obtained by means of numerical simulations. The numerical simulations for the two diblock systems predict structures with classic morphologies like bcc, hex, lamellar or gyroids and also other partial structure like islands and labyrinths. Young, bulk and shear modulus were also predicted from the structure and composition of the copolymer generating these morphologies. The excellent agreement between numerical and available experimental results opens a new strategy to modify existing diblock copolymer synthetic chemical processes to obtain products with specific morphologies.     

A scheme to predict the morphology of diblock copolymers and their blends

Diblock copolymers of α-methylstyrene and isoprene were synthesized anionically. The morphology of the copolymers and of their blends with the homopolymers was studied by transmission electron microscopy. Based on this, a scheme is proposed to predict the morphological behavior associated with the blending of block copolymers with homopolymers. Two blending systems are discussed. They are (i) copolymer AB with homopolymers A and B and (ii) copolymers AB of two different molecular weights with homopolymer A. Two factors are considered to be the most crucial. One is the morphology of the predominant polymer (50 wt %), and the other is the weight ratio of the blends. The solubilizing effect of the block copolymer AB in the blend must also be taken into account. When the two copolymers are blended, the one with lower molecular weight was emulsified by higher one and restricted around the longer chain. It is shown that the present scheme is successful in predicting the morphology of diblock copolymers and their blends.     

Crystallization behavior of oxyethylene/oxybutylene diblock and triblock copolymers

The crystallization behavior of poly(oxyethylene)-b-poly(oxybutylene) block copolymers with different compositions, morphologies and architectures (E_mB_n diblock copolymers and E_mB_nE_m, B_nE_mB_n triblock copolymers) were investigated and the effect of volume fraction and architecture on the crystallization temperature (T_c) in non-isothermal crystallization was determined. It is found that the E_mB_n diblock copolymers having long E blocks exhibit similar crystallization temperatures, irrespective of volume fraction and morphology, but for the block copolymers with shorter E blocks the crystallization temperature increases with both the volume fraction, φ_E, and the length, m, of the E block. Some block copolymers with extremely low T_c, which fall into the temperature range normally associated with homogenous nucleation, were chosen for time-resolved small-angle X-ray scattering (SAXS) and isothermal crystallization kinetics experiments. The results show that breakout crystallization occurs in all these block copolymers. Therefore, unlike E_mB_n/B_h blends, there is no obvious relationship between T_c and crystallization behavior in neat block copolymers and homogeneous nucleation does not definitely lead to confined crystallization. The values of χ_c/χ_ODT for all the block copolymers with hex and bcc morphology were also calculated. It is found that all the block copolymers have χ_c/χ_ODT<3, in agreement with the previously reported critical value and consistent with their breakout crystallization behavior.     

The influence of diblock copolymers on the structure and properties of polybutadiene-polyisoprene blends

The structure and properties of a ternary polymer system comprised of 1, 4 polybutadiene, cis 1, 4 polyisoprene, and the corresponding 1, 4 polybutadiene/cis 1, 4 polyisoprene diblock copolymer have been investigated. Dynamic mechanical properties were measured as a function of frequency (3.5 to 110 Hz) and temperature (?135 to 40°C). Thermomechanical analysis and transmission electron microscopy provided additional information on the, phase relationships in the various specimens. The blends studied covered the entire triangular composition diagram; both slow solvent evaporation and rapid spin casting techniques were employed in the sample preparation. Results indicate that the BI diblock is a single phase material whereas binary blends of the two homopolymers are two-phase in nature. Ternary blends of the two homopolymers with diblock and binary blends of a single homopolymer with diblock can be one or two phase materials depending on the sample composition and the ratio of B to I units in the diblock. All evidence From the dynamic, thermomechanical and microscopic experiments is used to elucidate the influence of the diblock in these polymer blends.     

Rod-rod and rod-coil self-assembly and phase behavior of polypeptide diblock copolymers

Poly(γ-benzyl l-glutamate) (PBLG) forms a rigid helical rod in organic solvents. Cholesteric liquid crystalline ordering of these rods has been observed in PBLG solutions and cast films. In this research, peptidic block copolymers were created using PBLG in order to determine the effect of an added block on the classic cholesteric ordering. Peptide blocks with varied lengths and inherent secondary structures, random coil or rigid rod, were attached to PBLG molecules. The self assembly/liquid crystalline ordering of these molecules in films cast from various organic solvents was probed with transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). In pure PBLG and PBLG diblock copolymers with relatively small additional blocks, cholesteric liquid crystalline ordering was observed in bulk films. However, depending on the kinetics of film formation and the amount of non-PBLG block, significant changes in the nanostructure and microstructure were observed. These purely peptidic block molecules provide the opportunity to pattern materials with peptidic functionalities by taking advantage of block copolymer phase behavior and liquid crystal ordering.     

New water soluble azobenzene-containing diblock copolymers: synthesis and aggregation behavior

Homopolymers of azobenzene (azo) methacrylates with different substituents and their diblock copolymers with poly(2-(dimethylamino)ethyl methacrylate p(DMAEMA) were synthesized via atom transfer radical polymerization (ATRP). Controlled/‘living’ ATRP of azo methacrylates were achieved up to ∼50% conversion, after which deviation occurred. It was found that the copolymerization rate of 6-[4-phenylazo]phenoxy]hexylmethacrylate (PPHM) from p(DMAEMA) macroinitiator was almost identical to that for the homopolymerization of PPHM monomer, with k_app∼0.0078 min⁻¹. For the copolymerizations, almost complete incorporation of the azo methacrylate monomers could be obtained with low molecular weight macroinitiator (PDMAEMA)-Cl, whereas macroinitiators of long chain length did not give full conversion, most likely due to chain floding and steric hindrance caused by the bulk azo monomers. Because azo monomers are highly hydrophobic, only the diblock copolymers with short azo segment were soluble in water which self-assembled into micellar particles. The effect of photo-induced trans-cis isomerization on lower critical solution temperature (LCST) and surface tension were studied. The LCST of the diblock copolymers increased upon irradiation by UV light due to the cis conformers being more hydrophilic. However, the trans-cis isomerization had only small effect on the critical micelle concentration (cmc) and γ_cmc of azo methacrylate block copolymers, due to the formation of compact core of the micelles. The formations of core-shell micelles were established from LLS and TEM studies. All the three azo methacrylate amphiphilic block copolymers formed hard core-shell micelles with relatively small R_h values of 31 nm for p(DMAEMA₁₇₂-b-BPHM₇), 26 nm for p(DMAEMA₁₇₂-b-CPHM₇) and 32 nm for p(DMAEMA₁₇₂-b-PPHM₉). Whereas for the azo acrylate copolymer, p(DMAEMA₁₇₂-b-BPHA₆), large micelles with R_h∼78 nm with loose core was formed.     

Stereocomplex‐induced gelation properties of polylactide/poly(ethylene glycol) diblock and triblock copolymers

The ring‐opening polymerization of L ‐ or D ‐lactide was realized in the presence of dihydroxyl or monomethoxy poly(ethylene glycol) (PEG) with a number‐average molecular weight of 2000. The resulting low‐molar‐mass poly(L ‐lactide) (PLLA)/PEG and poly(D ‐lactide) (PDLA)/PEG triblock and diblock copolymers were characterized with nuclear magnetic resonance (NMR), differential scanning calorimetry, size‐exclusion chromatography, and X‐ray diffractometric analysis. Bioresorbable hydrogels were successfully prepared from aqueous solutions containing both copolymers because of interactions and stereocomplexation between the PLLA and PDLA blocks. Gelation was evaluated with the tube inverting method and rheological measurements. A phase diagram was realized with gel–sol transitions as a function of concentration. The rheological properties of the hydrogels were investigated under various conditions through changes in the copolymer concentration, temperature, time, and frequency. It was concluded that the hydrogels constituted a dynamic and evolutive system because of the continuous formation/destruction of crosslinks and degradation. Further studies are underway to elucidate the degradation behavior and the potential of these substances as drug carriers or cell culture scaffolds. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Controlled one-pot synthesis of pH-sensitive self-assembled diblock copolymers and their aggregation behavior

Diblock copolymers of t-butyl methacrylate (tBMA) and 2-(diethylamino)ethyl methacrylate (DEAEMA) were successfully synthesized by one-pot strategy via the atom transfer radical polymerization (ATRP). Kinetic results clearly demonstrated the controlled/‘living’ character of the polymerization. The zwitterionic block copolymers of poly(methacrylic acid-b-DEAEMA), obtained by hydrolysis of poly(tBMA-b-DEAEMA), showed pH-dependent reverse micellization behavior. Micellar aggregates formed from poly(MAA₃₀-b-DEAEMA₇₁), poly(MAA₆₈-b-DEAEMA₅₅) and poly(MAA₆₄-b-DEAEMA₄₄) had fairly low polydispersity index at both solutions of low pH of 2 and high pH of 12. Micelles formed at pH 2 were larger (R_h∼40-61 nm) with looser core due to hydration of the MAA. In the presence of simple electrolyte (0.3 mol dm⁻³ NaCl solution), the size of the micelles reduced by almost half while the aggregation number was little changed. This is attributed to the draining of the hydrated micellar core due to osmotic pressure. On the other hand, DEAEMA-core micelles formed at pH 12 were compact and much smaller (R_h∼14-22 nm). Addition of NaCl had only a small effect. The micellar size reduced only slightly due to the electrostatic screening effect and the aggregation number was almost unchanged.     

Effect of triblock copolymers on homogeneity,mechanical properties and swelling behavior of IIR/SBR rubber blends

The present research concerns with the preparation and characterization of isobutylene isoprene/butadiene–styrene rubber (IIR/SBR) blends with different blend ratios, in the presence and absence of styrene–isoprene–styrene (SIS) and styrene–isobutylene–styrene (SiBS) triblock copolymers to be tested as compatibilizers. Effect of the triblock copolymers on the blend homogeneity was investigated with the aid of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) measurements. Characterization of the rubber blends was conducted by measuring the physico-mechanical properties after and before thermal aging, in presence and absence of the triblock copolymers. In addition, weight swell % in toluene, motor oil and brake fluid of the rubber blend vulcanizates was assessed. The incorporation of SIS and SiBS triblock copolymers improved the homogeneity of IIR/SBR blends as well as increased both tensile strength and elongation at break of the rubber blend vulcanizates. Of the entire blend ratios examined, IIR/SBR (25/75) blend containing SIS compatibilizer possessed the best physico-mechanical properties (12.6 MPa tensile strength and 425 % elongation at break) and (14 MPa tensile strength and 555 % elongation at break) after and before thermal aging, respectively. Utilization of SIS and SiBS triblock copolymers enhanced the thermal stability of IIR/SBR blend vulcanizates. Moreover, IIR/SBR blends of different blend ratios showed superior swelling resistance in the brake fluid. IIR/SBR (25/75) blend containing SIS compatibilizer and cured with CBS/ZDEC/S vulcanizing system possessed the best physico-mechanical properties (14.4 MPa tensile strength and 440 % elongation at break) and (16.5 MPa tensile strength and 610 % elongation at break) after and before thermal aging, respectively.     

Polymer blends with a crystallizable diblock copolymer: Thermal properties and phase behavior

We report on the thermal behavior of polymer blends comprising a block copolymer and random copolymers of styrene and acrylonitrile (SAN). The block copolymer constituents are ε-caprolactone (CL) and a carbonate. Melting as well as crystallization behavior of the blocks are strongly influenced by addition of amorphous SAN, which is miscible only with PCL block. Blends under discussion can undergo different phase changes as macro- and microphase separation and crystallization. Hence, morphology formation is controlled by different transitions and even a cooperative interplay between them. Phase behavior of the blends is discussed and correlated with results of morphological studies.     

Binary blends of nylons with ethylene vinyl alcohol copolymers: Morphological,thermal, rheological,and mechanical behavior

Binary blends of ethylene vinyl alcohol copolymers, containing 62 (EVOH-62) and 71 (EVOH-71) mole percent vinyl alcohols, with nylons (nylon-6, nylon-6/12, and nylon-12) have been prepared from melt mixing in a twin screw compounding machine. Morphological, thermal, rheological, and mechanical properties were determined. EVOH-62/nylon-6 and EVOH-71/nylon-6 blends showed homogeneous phase morphologies in the nylon-6-rich region, and fine phase separations (c.a. 2 × 10^?7 m) in the EVOH-rich region. Melting point depression, positive deviations in viscosity and flexural modulus, and negative deviation in impact strength from the simple additive rule were generally observed. And the results were possibly interpreted in terms of compatibility and increased nylon/EVOH interactions over the nylon/nylon interactions. On the contrary, clean phase separations in large domains were observed from EVOH-71/nylon-6/12 and EVOH-71 /nylon-12 blends. Fibrillation was also obtained from EVOH rich blends. Probably due to the incompatible nature of these blends, yield at low rate of shear and a mechanical property drop were also observed.     

Dynamic mechanical behavior of copolymers containing carbon black

The storage and loss moduli of random copolymers of styrene and butyl methacrylate containing carbon black of varied surface area were determined by dynamic mechanical analysis at several temperatures about 100°C above the glass-transition temperature, T_g. At low frequencies, the pure polymers exhibit linear double log plots of moduli against frequency, with slopes of unity and approaching two for G″ and G′, respectively. With the addition of carbon black filler, both G′ and G″ become independent of frequency and temperature at low frequencies, consistent with yield behavior arid the formation of a carbon black network. The limiting dynamic complex modulus exceeds the yield stress from steady shear rheology, perhaps indicating the extent of the carbon black network, which was highest for low-molecular-weight copolymer and polystyrene. The filled random copolymers behaved Theologically like similarly filled polystyrenes of comparable molecular weights. Plasticization effects observed in the steady shear rheology of filled copolymers containing small concentrations of carbon black were not observed in dynamic mechanical analysis, although dynamic moduli converge at high frequency.     

Modification of polylactide upon physical properties by solution-cast blends from polylactide and polylactide-grafted dextran

Polylactide (PLA)-grafted polysaccharides with various lengths and numbers of graft chains were synthesized using a trimethylsilyl protection method. The properties of the cast films prepared from graft-copolymers were investigated through thermal and dynamic mechanical analyses. The graft-copolymer films exhibited a lower glass transition temperature (T_g), melting temperature, and crystallinity, and higher viscosity properties compared to PLA films. Moreover, the usefulness of graft-copolymer as a plasticizer was investigated with 1:4 blend films prepared from the graft-copolymers and PLA. The blend films showed lower T_g and crystallinity, and higher viscosity properties compared to PLA films.     

LDPE/POE共混物的结晶行为和力学性能

采用熔融共混法制备了低密度聚乙烯（LDPE）／聚烯烃弹性体（POE）共混物,通过差示扫描量热法和广角X射线衍射表征了共？昆物的结晶行为及LDPE、POE结晶的相互影响,研究了共混物的力学性能。随着POE用量的增加,LDPE的结品度稍有减小,结晶的完善性和均一性变差,晶粒尺寸逐渐变小;LDPE在结晶过程中出现了二次结晶;共混物的硬度、定伸应力和熔体流动速率均逐渐减小,拉伸断裂应变则逐渐增加。当w（POE）为30％时,材料的拉伸强度达到最大值,为21.5MPa。随着LDPE含量的增加,POE的结晶度逐渐减小。     

Synthesis and micellization behavior of chiral amphiphilic diblock copolymers bearing amino acid/dipeptide pendants

Novel chiral amphiphilic diblock copolymers bearing l-phenylalanine or α-l-aspartyl-l-phenylalanine methyl ester pendants were synthesized using a “click” reaction. The structure and composition of copolymers were characterized by gel permeation chromatography, ¹H NMR, elemental analysis and optical rotation measurements, which showed that the synthetic route can produce copolymers with well-defined composition and optical activity. The results from surface tension measurements, dynamic light scattering, transmission electron microscopy and ¹H NMR spectra indicated that these chiral copolymers formed spherical micelles with an optically active hydrophobic core in a NaH₂PO₄ buffer (pH 4.5) system.     

Dynamic mechanical properties and morphology of polypropylene block copolymers and polypropylene/elastomer blends

The relation between the dynamic mechanical properties and the morphology of polypropylene (PP) block copolymers and polypropylene/elastomer blends was studied by dynamic mechanical analysis (DMA), light- and electron microscopy. The latter techniques contributed to an improvement in assignments of relaxation transitions in the DMA spectra. It was established that PP block copolymers had multiphase structure since the ethylene/propylene rubber phase (EPR) formed in the copolymerization contained polyethylene (PE) domains. An identical morphology was found in the case of PP/polyolefin thermoplastic rubber (TPO) blends. Impact modification of PP by styrene/butadiene block copolymers led to a multiphase structure, too, due to the polystyrene (PS) domains aggregated in the soft rubbery polybutadiene phase. In the semicrystalline polyolefinic and in the amorphous styrene/butadienebased thermoplastic rubbers, PE crystallites and PS do mains acted as nodes of the physical network structure, respectively. PP/EPDM/TPO ternary blends developed for replacing high-density PE showed very high dispersion of the modifiers as compared to that of PP block copolymers. This fine dispersion of the impact modifier is a basic regulating factor of impact energy dissipation in the form of shear yielding and crazing.     

The cocrystallization behavior of binary blends of isotactic polypropylene and propylene-ethylene random copolymers

Rapidly crystallized blends of metallocene isotactic polypropylene and propylene-ethylene random copolymers with an ethylene content varying from 0.76 to 7 mol% were found to cocrystallize to different degrees depending on the composition of the comonomer and content of copolymer in the blend. The degree of molecular mixing was studied using differential scanning calorimetry and solvent extraction techniques. A high extent of cocrystallization is obtained in the whole composition range of blends with a copolymer having up to ～ 2 mol% of ethylene. The degree of cocrystallization decreases gradually with increasing ethylene content or with increasing copolymer content in the blend. It is found that for ethylene contents as high as 5–7 mol% the copolymer rich blends show partial separate crystallization of the propylene ethylene copolymer. Thus, these crystals were selectively extracted at temperatures just above the dissolution temperature of the pure copolymer. In these blends, the fractional content of segments from the copolymer molecules incorporated in the cocrystal is low, yet it prevents extraction of these molecules at temperatures above the dissolution temperature of the copolymer. The degree of cocrystallization is explained by differences in crystallization kinetics of the pure components. The percentage of extracted material was found to be directly related to the dissolution temperature of the cocrystal which was also found to be a linear function of the inverse of the crystallite thickness. The high extent of cocrystallization observed for these polypropylene blends contrasts with comparable blends of polyethylenes. The blends of linear PE with a copolymer of 4 mol% branch units and higher, form separate crystals even after rapid crystallization.