Search results for "PLASTICS"

Determination of the glass transition temperature of poly(cyclohexyl acrylate) from oxygen permeability measurements

1993

Abstract The glass transition temperature ( T g ) of poly(cyclohexyl acrylate) (PCA) has been obtained from measurements of the oxygen permeability ( P = 0.60 barrers, T g = 23.5 ± 2.0°C) using a potentiostatic electrochemical sensor, and from the oxygen diffusion coefficient ( D = 0.12 × 10 −8 cm 2 s −1 , T g = 24.0 ± 1.8°C) by the time-lag method. A T g of 25 ± 1°C was found by differential scanning calorimetry and a T g of 25.0 ± 3.5°C was obtained from the specific volume by dilatometry. The fractional free volume ( v f ) at T g is 0.020 ± 0.002, which is slightly below (∼ 20%) most glassy polymers. A linear correlation has been observed between ln D and the reciprocal of v f , which su…

Fragmentation of polycarbonate macroinitiators in solution and in the bulk state

1990

The diphenylethane initiator 3,4-diethyl-3,4-(p-methoxyphenyl)-hexane (T–OCH3) was synthesized. By cocondensation of bisphenol A or C and the bisphenol derivative (“T”) of T–OCH3 with phosgene, polycarbonate macroinitiators PC(TxA1-x) and PC(TxC1-x) with different compositions x were made. The thermal fragmentation of T–OCH3 and the copolycarbonates was investigated with and without a radical scavenger both in solution and in the bulk state. The primary dissociation of T–OCH3 is a monomolecular Arrhenius process with a characteristic temperature of T* (τ = 1h) = 140°C (τ: average life time of the intact molecule). When the radicals are not intercepted, the dissociation is excessively revert…

The Exponential Dichotomy under Discretization on General Approximation Scheme

2011

This paper is devoted to the numerical analysis of abstract parabolic problem 𝑢  ( 𝑡 ) = 𝐴 𝑢 ( 𝑡 ) ; 𝑢 ( 0 ) = 𝑢 0 , with hyperbolic generator 𝐴 . We are developing a general approach to establish a discrete dichotomy in a very general setting in case of discrete approximation in space and time. It is a well-known fact that the phase space in the neighborhood of the hyperbolic equilibrium can be split in a such way that the original initial value problem is reduced to initial value problems with exponential decaying solutions in opposite time direction. We use the theory of compact approximation principle and collectively condensing approximation to show that such a decomposition o…

Understanding sigma-phase precipitation in a stabilized austenitic stainless steel (316Nb) through complementary CALPHAD-based and experimental inves…

2014

Abstract Sigma-phase precipitation in a 316Nb “stabilized” austenitic stainless steel was studied through complementary CALPHAD-based and dedicated experimental investigations. Thermokinetic calculations performed using Thermo-Calc (with the DICTRA module) and MatCalc software showed that the sigma phase (σ) precipitated directly at γ-austenite grain boundaries (GB) via a common solid-state reaction when carbon and nitrogen contents fell below a critical threshold. Residual δ ferrite was found to be more susceptible to σ-phase precipitation; this type of precipitation occurred via two mechanisms that depended on the concentration profiles of δ-ferrite stabilizing elements induced by previou…

Generic Method for Modular Surface Modification of Cellulosic Materials in Aqueous Medium by Sequential Click-Reaction and Adsorption

2012

A generic approach for heterogeneous surface modification of cellulosic materials in aqueous medium, applicable for a wide range of functionalizations, is presented. In the first step, carboxymethyl cellulose (CMC) modified with azide or alkyne functionality, was adsorbed on a cellulosic substrate, thus, providing reactive sites for azide–alkyne cycloaddition click reactions. In the second step, functional units with complementary click units were reacted on the cellulose surface, coated by the click-modified CMC. Selected model functionalizations on diverse cellulosic substrates are shown to demonstrate the generality of the approach. The concept by sequentially combining the robust physic…

Amphiphilic Polysaccharide Block Copolymers for pH-Responsive Micellar Nanoparticles

2017

A full polysaccharide amphiphilic block copolymer was prepared from end group-functionalized dextrans using copper-mediated azide-alkyne click chemistry. Sufficient modification of the reducing end in both blocks was achieved by microwave-enhanced reductive amination in a borate-buffer/methanol solvent system. The combination of a hydrophilic dextran block with a hydrophobic acetalated dextran block results in an amphiphilic structure that turns water-soluble upon acid treatment. The material has a low critical micelle concentration and self-assembles in water to spherical micellar nanoparticles. The formed nanoparticles have a narrow size distribution below 70 nm in diameter and disassembl…

HPMA-Based Nanocarriers for Effective Immune System Stimulation.

2019

The selective activation of the immune system using nanoparticles as a drug delivery system is a promising field in cancer therapy. Block copolymers from HPMA and laurylmethacrylate-co-hymecromone-methacrylate allow the preparation of multifunctionalized core-crosslinked micelles of variable size. To activate dendritic cells (DCs) as antigen presenting cells, the carbohydrates mannose and trimannose are introduced into the hydrophilic corona as DC targeting units. To activate DCs, a lipophilic adjuvant (L18-MDP) is incorporated into the core of the micelles. To elicit an immune response, a model antigen peptide (SIINFEKL) is attached to the polymeric nanoparticle-in addition-via a click rea…

1994

Liquid crystalline side chain polymers containing photoreactive azobenzene chromophores in the dark-adapted trans form and phenylbenzoate groups as co-mesogens have been prepared and investigated as monomolecular layers at the water/air interface and as multilayer assemblies deposited onto solid supports by the Langmuir–Blodgett–Kuhn (LBK) technique. Pressure area isotherms at variable temperatures together with reflection absorption spectra and Brewster angle microscopic pictures have been taken and analyzed (i) to derive information about the structural order and the packing properties of the mesogenic side groups within the monolayers of different homo- and co-polymeric systems and ii) t…

Fourier-transform infrared study of the switching process in a ferroelectric liquid crystalline polymer

1995

The implementation of the step-scan technique on our spectrometer enabled us to follow the electric field induced reorientation dynamics of different molecular segments of the ferroelectric liquid crystalline polymer [Si(Me)(R)O/Si(Me)(R')O/Si(Me) 2 O] n (R= (CH 2 ) 11 OPhCOOPhOCO * CHCl * CH(Me)(Et); R'= (CH 2 ) 11 OPhN=NPhO * CH(Me)(Hex)) on a sub-millisecond time scale. It was detected that not only the mesogen but also the spacer and at least part of the backbone take part in the reorientation process

Chiral Polyisocyanates from an Azomonomer with a Very High Chiral Induction

2001

The synthesis of new chiral polyisocyanates is described. For this purpose a new chiral azobenzene containing monomer with the chiral center in α-position to the isocyanate group was synthesized. The anionic copolymerization was carried out in THF as solvent with potassium cyanide that was complexed by 18-crown-6 as initiator. This allowed a better control of the reaction and thereby the synthesis of polyisocyanates with a fairly low polydispersity. The copolymers show an extremely high transfer of chirality from the chiral side groups to the helical backbone in dilute solution. Copolymers with only 1.6 mol % of chiral side groups show nearly the full optical rotation and exist predominantl…