No abstract

No abstract

ABSTRACT:The thermoresponsive conductive composite (TCC) thin films and fiber mats, whose electrical property changed with temperature, were fabricated successfully. The thermocrosslinkable and thermoresponsive copolymer, poly (N-isopropyl acrylamide-co-N-methylolacrylamide) (PNN), was synthesized. The TCC thin film and fiber mat were fabricated by spin coating and electrospinning process of PEDOT:PSS/ PNN solutions, respectively. After thermocrosslinking and doping by DMSO, the composite thin films and fiber mats were obtained. Fibrous structures of TCC fiber mats were observed by SEM. The surface resistance and conductivity of composites were measured. The thermoresponsivity and swelling ratio of TCCs were also studied. The thermoresponsive conductive property was analyzed by measuring the surface resistance of TCCs in water bath under various temperatures from 20 to 50 8C. With the increase of temperature, the TCCs shrank to be dense structure and showed lower surface resistance. The TCC fibers mat exhibited greater sensitivity to temperature than thin film owing to its fibrous structure.

Antiplasticization behavior was found in the polycaprolactone (PCL)/polycarbonate (PC)‐modified epoxy system, cured with an aromatic amine. The initial modulus increased and the fracture toughness and the elongation at break decreased with the addition of the PCL/PC modifier. The glass transition temperature (Tg) decreased slightly. In this system, the antiplasticization phenomenon can be explained well by the formation of hydrogen bonding between the carbonyl groups in the PCL/PC and the hydroxyl groups in the epoxy. The hydrogen bonding proportion, as analyzed from Fourier transform infrared spectra, increased with the addition of PCL/PC up to 15 parts. This is consistent with the trend observed in the mechanical properties. It was thought that for antiplasticization to occur, a strong molecular interaction is necessary for a restriction of molecular motion, in turn decreasing the free volume of the matrix and thereby causing an increase in the modulus of the material. The dynamic mechanical thermal behavior of these systems was also studied. It was found that there was no change in the molecular weights between crosslinks, which excluded the effect of crosslink density on the changes of the properties. Instead, the activation energy was increased and the peak area was decreased for the β relaxation because of the restriction of the molecular motion by hydrogen bonding. The results indirectly support the hypothesis that the motion of the 2‐hydroxypropyl ether is responsible for the β relaxation process.

In our previous article, we established that polycarbonate (PC) can react with the diglycidyl ether of bisphenol-A (DGEBA) at 200ЊC through transesterification and addition reactions, resulting in degraded PC chains with phenolic end groups and also in PC/DGEBA copolymers. However, these reactions can be minimized or eliminated at lower temperatures, below 160ЊC. In this article, Fourier transform infrared analysis (FTIR) was used to study the curing kinetics of epoxies in the presence of PC. The curing agent was an aromatic amine, diaminodiphenyl methane (DDM). FTIR results showed that the presence of a small amount of PC promoted the amineepoxide reactions, probably due to the catalytic effect of the phenolic end groups in the PC chains. However, the PC did not alter the epoxy cure reaction mechanism. Two different blending processes were used to premix the PC and DGEBA, namely, solutionblending and melt-blending processes, in order to give different extents of prereactions. If a solution-blending process was used, PC tended to undergo crystallization during an early stage of cure. When a melt-blending process was used, no melting peak was observed in the thermograms of the differential scanning calorimeter (DSC) for the modified epoxies; PC chains bonded to DGEBA during prereaction at 200ЊC, thus inhibiting the crystallization of PC during cure.

A series of environmentally sensitive ABA triblock copolymers with different block lengths were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization from acrylic acid (AA) and N-isopropylacrylamide (NIPAAm). The GPC and 1 H NMR analyses demonstrated the narrow molecular weight distribution and precise chemical structure of the prepared P(AA-b-NIPAAm-b-AA) triblock copolymers owing to the controlled/living characteristics of RAFT polymerization. The lower critical solution temperature (LCST) of the triblock copolymers could be tailored by adjusting the length of PAA block and controlled by the pH value. Under heating, the triblock copolymers underwent self-assemble in dilute aqueous solution and formed nanoparticles revealed via TEM images.Physically crosslinked nanogels induced by inter-/intra-hydrogen bonding or core-shell micelle particles thus could be obtained by changing environmental conditions. With a well-defined structure and stimuli-responsive properties, the P(AA-bNIPAAm-b-AA) copolymer is expected to be employed as a nanocarrier for biomedical applications in controlled-drug delivery and targeting therapy.

No abstract

Microporous PVDF membranes were prepared by immersion‐precipitation in 1‐octanol of casting dopes dissolved at different temperatures, with dissolution temperature affecting strongly the membrane microstructure. The effect of postcoagulation thermal annealing, which is an additional thermal parameter, on membrane microstructure and properties is probed herein. Membranes obtained were annealed at temperatures up to 160 °C, which is close to the melting point of PVDF polymer. Annealing leads to a substantial modification of the nano‐scale fine structure of the membranes, while the overall‐microporous structure is preserved. At elevated annealing temperatures, nano‐grains, fibrils, and stick‐like crystalline entities gradually eclipse, while globules develop more robust connections based on wide bands of crystal elements. Probing by X‐ray diffraction and dynamic scanning calorimetry shows that crystallinity increases when annealing temperature and time are increased. As regards mechanical properties, the tensile strength of the membranes can be enhanced substantially, up to about 10 times, upon appropriate high temperature prolonged annealing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1880–1893, 2009