Polyisobutylene model elastomers prepared using demonstrably complete end-linking reactions

Summary Cationic polymerizations with a trifunctional initiator-chain transfer agent were used to prepare three-arm polyisobutylene [C(CH₃)₂CH₂] (PIB) molecules with hydroxyl groups at all three chain ends. Extensive spectroscopic analyses confirmed the essentially perfect trifunctionality of the polymers, which were then end-linked using an aromatic diisocyanate to give trifunctional model networks. The PIB elastomers were found to have negligible sol fractions, which demonstrates that the end-linking reactions used to prepare them were essentially complete. They were studied, swollen, with regard to their equilibrium stressstrain isotherms in uniaxial extension at 25°C. As was found to be the case for trifunctional and tetrafunctional PIB networks prepared from the linear chains, the results were in satisfactory agreement with theory and yielded no evidence that inter-chain entanglements contribute to the modulus at elastic equilibriums.     

Novel amphiphilic membranes of poly(N,N-dimethyl acrylamide) crosslinked with octa-methacrylate-telechelic polyisobutylene stars

Summary A series of novel amphiphilic networks were synthesized by free-radical mediated copolymerization/crosslinking of N,N-dimethyl acrylamide (DMAAm) with various molecular weight octa-methacrylate telechelic polyisobutylene stars (PIB(MA)₈) as crosslinking agents. The overall composition of the PDMAAm-l-PIB(MA)₈ networks were controlled by controlling the concentration of the starting materials. Crosslinking was essentially complete as indicated by negligible sol fractions in both methanol and n-hexane. Swelling ratios and swelling rates of various PDMAAm-l-PIB(MA)₈ in water and n-heptane were determined and contrasted with those of networks prepared with di- and tri-methacrylate-telechelic PIB croslinkers (PIB(MA)₂ and PIB(MA)₃). The mechanical properties of water-swollen tubules made of PDMAAm-l-PIB(MA)₈ were studied and compared with tubules made with PIB(MA)₂ and PIB(MA)₃. Received: 30 January 2002 / Revised version: 6 May 2002 / Accepted: 7 May 2002     

Adhesive Failure and Deformation Behaviour of Polymers

An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress-strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.     

Polyisobutene-poly(methylmethacrylate) interpenetrating polymer networks: synthesis and characterization

Interpenetrating polymer networks (IPNs) combining polyisobutene (PIB) and poly(methyl methacrylate) (PMMA) networks were prepared using a in situ strategy. PIB networks were formed by isocyanate—alcohol addition between the hydroxyl end groups of telechelic dihydroxy-polyisobutene and an isocyanate cross-linker, catalyzed by dibutyltindilaurate (DBTDL). PMMA networks were obtained from free-radical co-polymerization of methyl methacrylate (MMA) with ethylene glycol bismethacrylate (EGDM) in the presence of dicyclohexyl peroxydicarbonate (DCPD) as the initiator. The synthesis of each network during the IPN formation was followed by FTIR spectroscopy. The highest degree of interpenetration is obtained by forming the PIB network first. The corresponding transparent IPNs exhibit two mechanical relaxation temperatures as determined by Dynamic mechanical thermal analysis (DMTA), corresponding to those of PMMA and PIB enriched phases. Mechanical properties of PIB networks are tremendously improved by the presence of PMMA network in such IPN architecture.     

Adhesive Failure and Deformation Behaviour of Polymers

An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress–strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.     

Polyisobutene/polycyclohexyl methacrylate interpenetrating polymer networks

Interpenetrating polymer networks (IPNs) combining polyisobutene (PIB) and poly(cyclohexyl methacrylate) (PCHMA) networks were prepared using an in situ strategy. PIB networks were formed by alcohol-isocyanate addition between the hydroxyl end groups of telechelic dihydroxypolyisobutene and an isocyanate cross-linker, catalyzed by dibutyltindilaurate (DBTDL). PCHMA networks were obtained from free-radical copolymerization of cyclohexyl methacrylate (CHMA) with ethylene glycol bismethacrylate (EGDM) in the presence of dicyclohexyl peroxydicarbonate (DCPD) as the initiator. The network formations into the IPN architecture were followed by FTIR spectroscopy. In a large composition range, transparent IPNs exhibit two mechanical relaxation temperatures as determined by dynamic mechanical thermal analysis (DMTA), corresponding to those of a PIB enriched phase and of one interpenetrating phase containing the PCHMA network. This morphology was confirmed by IPN surface analysis by AFM. As expected, mechanical properties of PIB networks are improved by the presence of PCHMA network in such IPN architectures.     

Adhesive Fracture Energies of Some High Performance Polymers

A fracture mechanics approach was applied to determine the adhesive fracture energy of various high performance polymers. These polymers, including both thermosetting and thermoplastic materials, generally offer higher temperature capability than conventional epoxies. Double tapered cantilever beam specimens were used for fracture tests at both room temperatures and 225°C. The adhesive fracture energies of a tetrafunctional epoxy and a phthalocyanine resin were also determined at low temperatures. Adhesive fracture behavior of polymers at high temperatures was found to depend on polymer glass transition temperature, whereas at low temperatures it was related to secondary relaxation processes in the glassy state.     

Adhesive Fracture Energies of Some High Performance Polymers

A fracture mechanics approach was applied to determine the adhesive fracture energy of various high performance polymers. These polymers, including both thermosetting and thermoplastic materials, generally offer higher temperature capability than conventional epoxies. Double tapered cantilever beam specimens were used for fracture tests at both room temperatures and 225°C. The adhesive fracture energies of a tetrafunctional epoxy and a phthalocyanine resin were also determined at low temperatures. Adhesive fracture behavior of polymers at high temperatures was found to depend on polymer glass transition temperature, whereas at low temperatures it was related to secondary relaxation processes in the glassy state.     

Synthesis and characterization of polyisobutylene propellant binders

Substantial gains in rocket motor cost-effectiveness or performance can be achieved through improved propellant aging characteristics or better and more reproducible mechanical properties. The approach to achieving these goals is the development of a new binder based on ideal networks formed from polyisobutylene (PIB) prepolymers. PIB has a greater resistance to oxidation than polybutadiene which is used in many current composite systems.The ideal networks are generated through the use of PIB prepolymers with controlled molecular weight distributions and well defined functionality. PIB prepolymers evaluated in this study were synthesized by Dr. J. P. Kennedy at the University of Akron using the inifer process which allows close control of molecular weight distribution, functionality and types of end group. Hydroxy-terminated PIB's with molecular weights that varied from 1500 to 6000 and functionalities of 2 or 3 were synthesized. After a thorough characterization, gumstocks were prepared with isocyanate curatives of controlled functionality. The effects of M_c and plasticizer content on mechanical properties were determined in addition to the aging characteristics of the systems. Accelerated aging studies of the gumstocks were conducted.This work was performed under Contract No. F04611-82-C-0066, AFRPL/MKPA Edwards AFB, CA, Project Manager -Lt. Joseph W Lewis     

Latex interpenetrating polymer networks based on acrylic polymers. II. The influence of the degree of network compatibility on morphology

Two latex interpenetrating polymer networks, one prepared from a pair of supposedly compatible polymers and the other from an incompatible pair, were investigated using transmission electron microscopy and dynamic mechanical analysis. From the results, it was proposed that both interpenetrating polymer networks consisted of latex particles with essentially coreshell morphologies. Evidence for a core-shell structure was more marked for the materials synthesised from the incompatible polymers. The other polymer pair showed indications of a significant amount of mixing.     

Crosslinking of polydimethylsiloxane diol blended with poly(dimethylsiloxane-b-styrene-b-dimethylsiloxane)

A variety of blends of poly(dimethylsiloxane-b-styrene-b-dimethylsiloxane) copolymers with hydroxyl terminated polydimethylsiloxane (molecular weight, 29 600 gmol^?1) were prepared. Crosslinking of blends was accomplished at room temperature by reacting hydroxyl chain ends with a tetrafunctional orthosilicate using dibutyltin dilaurate as a catalyst. The crosslink density was evaluated by toluene swelling. Catalyst concentration influences the crosslink density as observed from the amount of extractables in toluene. The tensile strength of crosslinked polydimethylsiloxane was found to increase by blending with block polymers or with silica fillers. Structural uniformity of fractured surfaces of rubbers was also studied by scanning electron microscopy.     

Macroporous rubber gels as reusable sorbents for the removal of oil from surface waters

Macroporous organogels were prepared by solution crosslinking various rubbers in benzene at ?18 °C. Butyl rubber (PIB), cis-polybutadiene (CBR) and styrene–butadiene rubber (SBR) were used as the rubber components, while sulfur monochloride was the crosslinker in the gel preparation. The organogel networks consist of large pores of 10¹–10² μm in size caused by the benzene crystals acting as a template during gelation. The networks formed by CBR and SBR showed an aligned porous structure consisting of regular pores, whereas those derived from PIB had irregular pores with a broad pore size distribution due to the phase separation of PIB chains at low temperatures. All organogels were very tough and could be completely compressed without any crack development. Sorption tests showed that the organogels were efficient at removing crude oil, gasoline, diesel, fuel oil and olive oil. The organogels are reusable once they are squeezed, leading to continuous sorption capacities of CBR or SBR gels for crude oil and olive oil of 33–38 g/g and 24–27 g/g, respectively. These sorption capacities are two to three times the capacity of the gels derived from PIB.     

Synthesis of tetrafunctional epoxy resins and their modification with polydimethylsiloxane for electronic application

High-performance tetrafunctional epoxy resins were synthesized by reacting a suitable tetraphenols which were obtained by the condensation of appropriate dialdehyde with phenol followed by epoxidation with a halohydrin. The structure of the synthesized tetraphenols was confirmed by infrared (IR), mass spectra (MS), and nuclear magnetic resonance (NMR) spectroscopy. Dispersed silicone rubbers were used to reduce the stress of the synthesized tetrafunctional epoxy resin cured with phenolic novolac resin for electronic encapsulation application. The dynamic viscoelastic properties and morphologies of neat rubber-modified epoxy networks were investigated. The thermal mechanical properties and moisture absorption of encapsulants formulated from the synthesized tetrafunctional epoxy resins were also studied. The results indicate that a low-stress, high glass transition temperature (T_g), and low-moisture-absorbing epoxy resin system was obtained for semiconductor encapsulation application. © 1996 John Wiley & Sons, Inc.     

Preparation of hydrophobic poly(isobutylene) star polymers with hydrophilic poly(propylene imine) dendritic cores

Summary Poly(propylene imine) dendrimers with amino chain ends have been used for the preparation of hydrophobic star polymers with hydrophilic dendritic cores. The unsaturated end groups of polyisobutylene (PIB) were transformed into reactive anhydride end groups by an “ene” reaction with maleic anhydride, and the resulting functionalized PIB was then reacted with dendrimers to afford dendrimer-PIB star copolymers. Received: 20 May 1999/Accepted: 17 June 1999     

Synthesis,characterization, and properties of novel polyisobutylene-based urethane model networks

This paper concerns the synthesis, characterization, and physical properties of novel polyisobutylene (PIB)-based urethane model networks prepared from diphenylmethane diisocyanate (MDI) and three-arm star PIBs capped with ? CH₂OH end groups (PIB(CH₂OH)₃). The PIB(CH₂OH)₃ starting materials were produced by the inifer method in the M?_n = 13,550–27,000 range. The best networks were obtained with NCO : OH = 1. Solvent extractions showed uncatalyzed network formation to be essentially complete and swelling studies indicated the expected architecture, i.e., M?_c = 2M?_n/3 and virtual absence of dangling chains. Stannous octoate was found to increase the rate of network formation in the absence of side reactions (i.e., allophanate formation), while other catalysts also accelerated undesirable reactions. The effect of molecular weight between crosslinks (M?_c) on network physical properties has been studied in the M?_c = 900–18,500 range. The tensile strength increases with decreasing M?_c up to a limiting value after which it sharply declines. Elongations at break decrease monotonously with decreasing M?_c. Low temperature tensile studies show higher tensile data at ?20°C, and, surprisingly, a retention of elongations at break. The T_g's decrease with increasing M?_c's until a plateau is reached at ?73°C. Hysteresis is fairly constant and permanent set is constant and low. The PIB-based urethane networks exhibit excellent hydrolytic stability, negligible moisture absorption, and outstanding heat-aging stability, far beyond what is expected for a polyurethane. Conceivably the thermal deblocking of the urethane group may be reversible (? NH? COO? CH₂? ? ? NCO + HOCH₂? ) because the isocyanate that arises in the highly hydrophobic PIB matrix recombines with the alcohol, and cannot react with moisture as in conventional urethane networks.     

Diffusion of organic solvents in isobutylene-based polymers

Diffusion behavior of several organic solvents in polyisobutylene (PIB) and in poly(p-methylstyrene-co-iso-butylene) (PMS-BR) with different monomer ratios has been studied. The experiments have been conducted over a temperature range of 50 to 100°C using a conventional gravimetric Sorption technique. The PMS-BR copolymers contained 2, 7, and 15 weight percent p-methylstyrene, respectively. Although employing temperatures were far above the glass transition temperatures of polymers, the diffusion coefficients are correlated well with the Vrentas-Duda free-volume theory. For all the solvents, the PIB shows the highest diffusivity while the copolymer with the 15% p-methylstyrene gives the lowest value. This behavior can be explained by the amount of fractional free-volume present in a system.     

Synthesis of amphiphilic star block copolymers of polystyrene with PEG core via ATRP—control of chain architecture and the formation of core-shell type globular structure

Amphiphilic star block copolymers made of poly(ethylene glycol) (PEG) core and branched PS arms having controlled chain lengths and numbers were synthesized by atom transfer radical copolymerization (ATRP) of styrene and chloromethylstyrene (CMS) in the presence of tetrafunctional PEG macroinitiator. The chain lengths and number of PS chains were controlled by adjusting the initial feed ratio of CMS to styrene and CMS to hydrophilic tetrafunctional macroinitiator, respectively, for a given polymerization time. The obtained polymers have well defined and controlled architectures. Use of excess amount of CMS and longer reaction time leads to the synthesis of dendrimer like amphiphilic block copolymer having four hyperbranched polymer arms, whose shape is closer to globular core-shell structure compared to general star shape polymers.     

Hochmolekulare lösliche polymere mit coronandenstruktur-synthese und eigenschaften

New polymers with an alternating sequence of aliphatic chains and coronands were synthesized via Diels-Alder addition polymerization of bis(2-pyrones) with bismaleimides. The chemical structures of these polymers 6 are supported by NMR spectra, model reactions and crystal X-ray structure analysis. The tetrafunctional maleimide 5 was synthesized and the cyclization of 5 to the bicycle 9 with 2 mol monopyrone 8 was carried out.     

Polyisobutylene-based urethane foams. II. Synthesis and properties of novel polyisobutylene-based flexible polyurethane foams

Novel polyisobutylene-based flexible polyurethane foams (PIB–PUF) have been prepared manually by the prepolymer method using three-arm star hydroxyl-terminated polyisobutylenes (PIB–triols) and toluene diisocyanate (TDI). Solvent extraction and IR spectroscopy of PIB–PUFs indicated essentially complete crosslinking. Conventional polyether-based polyrethane foams (PE–PUFs) and polybutadiene-based polyurethane foams (PBD–PUFs) have also been prepared by the same method and select physical-mechanical properties of all these urethane foams, such as tensile strength, elongation, resilience, water permeability, hot air stability, and hydrolytic stability, have been examined and compared. Although the density of PIB–PUF is lower than that of PE–PUF, its tensile strength is superior to the latter. Elongation of PIB–PUF is almost the same as those of the other foams. The PIB–PUF exhibits low resilience which indicates good damping properties. Due to the hydrophobicity of the soft segment, PIB–PUF exhibits very low water permeability. The hydrolytic and hot air stability of PIB–PUFs are outstanding. Attempts have been made to determine gas permeabilities; however, due to the open-cell nature of the foams, these studies could not be completed. The new PIB-based urethane foams combine excellent thermal, environmental, barrier, and mechanical properties, unmatched by conventional PUFs.     

New precursors to SiCO ceramics derived from linear poly(vinylsiloxanes) of regular chain composition

Two linear polysiloxanes with vinyl groups regularly distributed in their chains (D₂V polymer with a vinyl group at every third and V₃ polymer with a vinyl group at each Si atom) were cross-linked with different hydrogensiloxanes (linear difunctional ^HMM^H, linear branched tetrafunctional Q(M^H)₄, cyclic tetrafunctional D₄^H) and studied as new precursors to SiCO ceramics. It was found that thermal properties of the cross-linked D₂V and V₃ polymers are governed mainly by functionality of the initial macromolecule as well as functionality and molecular structure of hydrogensiloxane. Transformation of the systems into ceramics was examined by recording FTIR spectra of the samples obtained after heat treatment of the cross-linked polymers under Ar at several temperatures. Analysis of the products formed at 1 000 °C, allowed establishing that they are a mixture of silicon oxycarbides and a free carbon phase. Lack of pores is a characteristic feature of these materials.