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Thermal and mechanical studies of carbon nanotube-polymer composites synthesized at high pressure and high temperature
Umeå University, Faculty of Science and Technology, Department of Physics.
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.
Identifiers
URN: urn:nbn:se:umu:diva-43477ISBN: 978-91-7459-223-8OAI: oai:DiVA.org:umu-43477DiVA: diva2:414460
Public defence
2011-06-03, Naturvetarhuset, N300, Umeå universitet, Umeå, 10:15 (English)
Opponent
Supervisors
Available from: 2011-05-06 Created: 2011-05-02 Last updated: 2011-05-04Bibliographically approved
List of papers
1. Crosslinking, thermal properties and relaxation behaviour of polyisoprene under high-pressure
Open this publication in new window or tab >>Crosslinking, thermal properties and relaxation behaviour of polyisoprene under high-pressure
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.

Keyword
High-pressure, crosslinking, thermal conductivity, glass transition
Identifiers
urn:nbn:se:umu:diva-11298 (URN)10.1016/j.eurpolymj.2008.07.010 (DOI)
Available from: 2008-12-09 Created: 2008-12-09 Last updated: 2011-05-05Bibliographically approved
2. Thermal conductivity, heat capacity, and cross-linking of polyisoprene/single-wall carbon nanotube composites under high pressure
Open this publication in new window or tab >>Thermal conductivity, heat capacity, and cross-linking of polyisoprene/single-wall carbon nanotube composites under high pressure
2009 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 42, no 23, 9295-9301 p.Article in journal (Refereed) Published
Abstract [en]

Polyisoprene (PI)/single-wall carbon nanotube (SWCNT) composites and pure PI have been cross-linked by high-pressure treatment to yield densified elastomeric states. Simultaneously, the SWCNT and cross-linked-induced changes of the thermal conductivity, heat capacity per unit volume, and glass transition were investigated by in situ measurements. The thermal conductivity of both the elastomeric and liquid PI improves ≈120% by the addition of 5 wt % SWCNT filler. The SWCNT filler (5 wt %) increases the glass-transition temperature of liquid PI by ≈7 K and that of the elastomeric state by as much as 12 K, which is due to a filler-induced increase in the cross-link density. Moreover, the 5 wt% filler yields a heat capacity decrease by ≈30% in both the glassy and liquid/elastomeric states, which indicates that SWCNTs cause a remarkably large reduction of both the vibrational and configurational heat capacity of PI. Finally, the consequences of high-pressure densification and the possibilities this provides to help elucidating the nature of the heat conduction in polymer/carbon nanotube composites are discussed.

Place, publisher, year, edition, pages
American Chemical Society, 2009
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-30449 (URN)10.1021/ma902122u (DOI)
Available from: 2009-12-25 Created: 2009-12-25 Last updated: 2011-11-21Bibliographically approved
3. Tensile strength and young's modulus of polyisoprene/single-wall carbon nanotube composites increased by high pressure cross-linking
Open this publication in new window or tab >>Tensile strength and young's modulus of polyisoprene/single-wall carbon nanotube composites increased by high pressure cross-linking
2010 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 43, no 18, 7680-7688 p.Article in journal (Refereed) Published
Abstract [en]

High-viscosity liquid cis-1,4 polyisoprene (PI), with up to 20 wt % single-wall carbon nanotubes (SWCNTs), has been cross-linked by high pressure and high temperature (HP&HT) treatment at 513 K and pressures in the range 0.5 to 1.5 GPa to yield densified network polymer composites. A composite with 5 wt % SWCNTs showed 2.2 times higher tensile strength σUTSUTS = 17 MPa), 2.3 times higher Young’s modulus E (E = 220 MPa) and longer extension at break than pure PI. The improvement is attributed to SWCNT reinforcement and improved SWCNT−PI interfacial contact as a result of the HP&HT cross-linking process, and reduced brittleness despite a higher measured cross-link density than that of pure PI. The latter may originate from an effect similar to crazing, i.e., bridging of microcracks by polymer fibrils. We surmise that the higher cross-link densities of the composites are due mainly to physical cross-links/constraints caused by the SWCNT−PI interaction, which also reflects the improved interfacial contact, and that the CNTs promote material flow by disrupting an otherwise chemically cross-linked network. We also deduce that the PI density increase at HP&HT cross-linking is augmented by the presence of CNTs.

Place, publisher, year, edition, pages
American Chemical Society, 2010
Identifiers
urn:nbn:se:umu:diva-38842 (URN)10.1021/ma101484e (DOI)000281883000035 ()
Available from: 2011-01-03 Created: 2011-01-03 Last updated: 2011-11-21Bibliographically approved
4. High-pressure-induced microstructural evolution and enhancement of thermal properties of nylon-6
Open this publication in new window or tab >>High-pressure-induced microstructural evolution and enhancement of thermal properties of nylon-6
2010 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 43, no 24, 10512-10520 p.Article in journal (Refereed) Published
Abstract [en]

The transition behavior and thermal properties of nylon-6 at elevated pressure, p, have been established by in-situ thermal conductivity, κ, and heat capacity measurements. The glass transition temperature, Tg, of virgin nylon-6 is described well by the empirical equation Tg(p) = 319.60(1 + 1.90 p)0.24 (p in GPa and Tg in K). Moreover, isobaric heating in the 1−1.2 GPa range causes a cold-crystallization transition near 500 K. As a result, κ increased 15% whereas the heat capacity per unit volume decreased 7% slowly with time during 4 h annealing at 530 K. The transformation is associated with a significantly increased crystallinity, from 35% to 55−60%, and a pressure-induced preferred orientation and increased size for the lamellae of monoclinic α crystalline structure. This state has 8−10 K higher melting temperature and better formic acid resistance than that of virgin nylon-6. However, the results show no indication of cross-linking, as reported for similarly treated nylon-1010 and nylon-11, but instead chain scissoring.

Identifiers
urn:nbn:se:umu:diva-38807 (URN)10.1021/ma102273b (DOI)000285429400049 ()
Available from: 2011-01-03 Created: 2011-01-02 Last updated: 2011-11-21Bibliographically approved

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