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Crosslinking, thermal properties and relaxation behaviour of polyisoprene under high-pressure
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2008 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 44, no 9, 2865-2873 p.Article in journal (Refereed) Published
Abstract [en]

The transient hot-wire method has been used to measure the thermal conductivity k and heat capacity per unit volume rcp of untreated (virgin) and crosslinked cis-1,4-poly(isoprene) (PI) in the temperature range 160–513 K for pressures p up to 0.75 GPa. The results show that the crosslinking rate of the polymer chains becomes significant at 513 K on isobaric heating at 0.5 GPa changing PI into an elastomeric state within 4 h without the use of crosslinking agents. The crosslinked PI and untreated PI have about the same k = 0.145 Wm-1 K-1 and cp = 1.81 kJ kg-1 K-1 at 295 K and 20 MPa, but different relaxation behaviours. Two relaxation processes, corresponding to the segmental and normal modes, could be observed in both PI and crosslinked PI but these have a larger distribution of relaxation times and become arrested at higher temperatures (10 K) in the latter case. The arrest temperature for the segmental relaxation of untreated and crosslinked PI, for a relaxation time of 1 s, are described well by the empirical relations: T(p) = 209.4 (1 + 4.02 p)^0.31 and T(p) = 221.3 (1 + 2.33 p)^0.40 (p in GPa and T in K), respectively, which thus also reflects the pressure variations of the glass transition temperatures.

Place, publisher, year, edition, pages
2008. Vol. 44, no 9, 2865-2873 p.
Keyword [en]
High-pressure, crosslinking, thermal conductivity, glass transition
URN: urn:nbn:se:umu:diva-11298DOI: 10.1016/j.eurpolymj.2008.07.010OAI: diva2:150969
Available from: 2008-12-09 Created: 2008-12-09 Last updated: 2011-05-05Bibliographically approved
In thesis
1. Thermal and mechanical studies of carbon nanotube-polymer composites synthesized at high pressure and high temperature
Open this publication in new window or tab >>Thermal and mechanical studies of carbon nanotube-polymer composites synthesized at high pressure and high temperature
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, thermal and mechanical properties of polymers and carbon nanotubes-polymer composites, which were modified and studied under high pressure, are presented. The results concern the thermal conductivity κ and heat capacity per unit volume ρcp of pure polymers: polyisoprene (PI), polybutadiene (PB), and nylon-6, and their multi-wall and single-wall carbon nanotube (MWCNT and SWCNT) composites both before (untreated) and after high pressure treatments. As shown here, a suitable high pressure high temperature (HP&HT) treatment induces either cross-links in the polymers (PI and PB), i.e. transforms these into elastomers, or increases the crystallinity (nylon-6).

The experiments were done, in situ, in the temperature range 100-520 K for pressures up to 1.5 GPa, and the results show that cross-linking under high pressure can be monitored in data for κ and ρcp. Moreover, κ for a well cross-linked (ebonite-like) polymer near ambient conditions can be up to 50% higher than the untreated states, whereas ρcp becomes similar as the glassy state of the untreated polymer. The glass transition of the cross-linked states becomes broader and shifts to higher temperatures with increasing degree of cross-linking. In the case of nylon-6, the HP&HT treatment causes microstructural changes, viz. increased crystallinity and crystals with a preferred orientation and increased size, which enhances κ and improves the thermal stability.

The thermal property studies of the CNT polymer composite show that k of the composites increases significantly, e.g. 120% for 5wt% SWCNTs in PI, which is attributed to the very high k of CNTs. Moreover, MWCNTs also improve k, but not as much as SWCNTs. This is accounted for by their lower aspect ratio (length/diameter), whereas their lower k is less important. Adding CNTs normally raise the glass transition temperatures of the polymers. More specifically, SWCNTs in PB raise the glass transition temperature slightly more than MWCNTs and, in particular, under the most densified conditions and for a high molecular weight PB, which may be due to more favorable conditions for coating/wrapping of the CNTs.

The mechanical studies of the HP&HT treated polymers and composites show that CNTs strongly enhances the tensile strength and Young’s modulus, e.g. 5 wt% SWCNT in PI synthesized at 1 GPa and 513 K showed 2 times higher tensile strength and 2.3 times higher Young’s modulus than that of similarly treated pure PI. The results indicate that the treatment improves the poor interfacial contact between the CNTs and polymer, which is one of the obstacles for achieving stronger CNT composites

Place, publisher, year, edition, pages
Umeå: Institutionen för fysik, Umeå Universitet, 2011. 52 p.
urn:nbn:se:umu:diva-43477 (URN)978-91-7459-223-8 (ISBN)
Public defence
2011-06-03, Naturvetarhuset, N300, Umeå universitet, Umeå, 10:15 (English)
Available from: 2011-05-06 Created: 2011-05-02 Last updated: 2011-05-04Bibliographically approved

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