| 1. |
- Dominguez-Rodrigo, Manuel, et al.
(författare)
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Constraining time and ecology on the Zinj paleolandscape : Microwear and mesowear analyses of the archaeofaunal remains of FLK Zinj and DS (Bed I), compared to FLK North (Bed I) and BK (Bed II) at Olduvai Gorge (Tanzania)
- 2019
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Ingår i: Quaternary International. - : Elsevier BV. - 1040-6182 .- 1873-4553. ; 526, s. 4-14
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Tidskriftsartikel (refereegranskat)abstract
- Defining the time that any given archaeofaunal assemblage took to be accumulated is challenging. Understanding the time variable is crucial to interpret how early sites were formed and what these sites represent in terms of hominin behavior. Two complementary dental analysis techniques (microwear and mesowear) have been used to understand dietary niches of ungulates. Microwear has also specifically been used to detect the character of occupations at archaeological sites. Here, we apply these techniques to a selection of Olduvai sites. Microwear and mesowear analyses on bovid teeth from a set of anthropogenic sites (FLK Zinj, DS, BK) and a carnivore palimpsest (FLK North) yielded different results. Microwear data from the three anthropogenic sites are similar, reflecting short, seasonal occupations, in contrast with the carnivore assemblage, which suggests a more prolonged period of deposition. The similar microwear signal in the two pene-contemporaneous sites of FLK Zinj and DS is encouraging, but caution in its interpretation is applied because of limited tooth enamel preservation and the resulting small sample size. The results shown here must be considered as a baseline for future and more extensive studies. Both microwear and mesowear analyses show that the most common bovid taxa in the Bed I sites exhibit mixed feeding signals. This reinforces the caution about interpretations of dietary niches of extinct ungulates based on their modern counterparts and emphasizes that for some taxa, the adoption of a browsing or grazing diet is context (time and locus) specific.
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| 2. |
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| 3. |
- Boujemaoui, Assya, et al.
(författare)
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Polycaprolactone Nanocomposites Reinforced with Cellulose Nanocrystals Surface-Modified via Covalent Grafting or Physisorption : A Comparative Study
- 2017
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 9:40, s. 35305-35318
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, cellulose nanocrystals (CNCs) have been surface-modified either via covalent grafting or through physisorption of poly(n-butyl methacrylate) (PBMA) and employed as reinforcement in PCL. Covalent grafting was achieved by surface-initiated atom transfer radical polymerization (SI-ATRP). Two approaches were utilized for the physisorption: using either micelles of poly(dimethyl aminoethyl methacrylate)-block-poly(n-butyl methacrylate) (PDMAEMA-b-PBMA) or latex nanoparticles of poly(dimethyl aminoethyl methacrylate-co-methacrylic acid)-block-poly(n-butyl methacrylate) (P(DMAEMA-co-MAA)-b-PBMA). Block copolymers (PDMAEMA-b-PBMA)s were obtained by ATRP and subsequently micellized. Latex nanoparticles were produced via reversible addition-fragmentation chain-transfer (RAFT) mediated surfactant-free emulsion polymerization, employing polymer-induced self-assembly (PISA) for the particle formation. For a reliable comparison, the amounts of micelles/latex particles adsorbed and the amount of polymer grafted onto the CNCs were kept similar. Two different chain lengths of PBMA were targeted, below and above the critical molecular weight for chain entanglement of PBMA (M-n,M-c similar to 56 000 g mo1(-1)). Poly(epsilon-caprolactone) (PCL) nanocomposites reinforced with unmodified and modified CNCs in different weight percentages (0.5, 1, and 3 wt %) were prepared via melt extrusion. The resulting composites were evaluated by UV-vis, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and tensile testing. All materials resulted in higher transparency, greater thermal stability, and stronger mechanical properties than unfilled PCL and nanocomposites containing unmodified CNCs. The degradation temperature of PCL reinforced with grafted CNCs was higher than that of micelle-modified CNCs, and the latter was higher than that of latex-adsorbed CNCs with a long PBMA chain length. The results clearly indicate that covalent grafting is superior to physisorption with regard to thermal and mechanical properties of the final nanocomposite. This unique study is of great value for the future design of CNC-based nanocomposites with tailored properties.
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| 4. |
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| 5. |
- Cobo Sanchez, Carmen
(författare)
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Inorganic and organic polymer-grafted nanoparticles : their nanocomposites and characterization
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanocomposites (NCs) have been widely studied in the past decades due to the promising properties that nanoparticles (NPs) offer to a polymer matrix, such as increased thermal stability and non-linear electrical resistivity. It has also been shown that the interphase between the two components is the key to achieving the desired improvements. In addition, polymer matrices are often hydrophobic while NPs are generally hydrophilic, leading to NP aggregation. To overcome these challenges, NPs can be surface-modified by adding specific molecules and polymers. In the present work, a range of organic and inorganic NPs have been surface-modified with polymers synthesized by atom transfer radical polymerization (ATRP) or surface-initiated ATRP (SI-ATRP).Cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) are highly crystalline NPs that can potentially increase the Young’s modulus of the NC. In this study, a matrix-free NC was prepared by physisorption of a block-copolymer containing a positively charged (quaternized poly(2-(dimethylamino)ethyl methacrylate), qPDMAEMA) and a thermo-responsive (poly di(ethylene glycol) methyl ether methacrylate, PDEGMA). The modified CNF exhibited a thermo-responsive, reversible behavior. CNCs were polymer-modified either via SI-ATRP or physisorbed with poly (butyl methacrylate) (PBMA) to improve the dispersion and interphase between them and a polycaprolactone (PCL) matrix during extrusion. The mechanical properties of the NCs containing CNC modified via SI-ATRP were superior to the reference and unmodified materials, even at a high relative humidity.Reduced graphene oxide (rGO) and aluminum oxide (Al2O3) are interesting for electrical and electronic applications. However, the matrices used for these applications, such as poly(ethylene-co-butyl acrylate) (EBA) and low density polyethylene (LDPE) are mainly hydrophobic, while the NPs are hydrophilic. rGO was modified via SI-ATRP using different chain lengths of PBMA and subsequently mixed with an EBA matrix. Al2O3 was modified with two lengths of poly(lauryl methacrylate) (PLMA), and added to LDPE prior to extrusion. Agglomeration and dispersion of the NCs were dependent on the lengths and miscibilities of the grafted polymers and the matrices. rGO-EBA NCs showed non-linear direct current (DC) resistivity upon modification, as the NP dispersion improved with increasing PBMA length. Al2O3-LDPE systems improved the mechanical properties of the NCs when low amounts of NPs (0.5 to 1 wt%) were added, while decreasing power dissipation on the material.Finally, PLMA-grafted NPs with high polymer quantities and two grafting densities in Al2O3 and silicon oxide (SiO2) nanoparticles were synthesized by de-attaching some of the silane groups from the surfaces, either by hydrolysis or by a mild tetrabutylammonium fluoride (TBAF) cleavage. These compounds were characterized and compared to the bulk PLMA, and were found to have very interesting thermal properties.
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| 6. |
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| 7. |
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| 8. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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DC electrical conductivity of LDPE-based nanocomposites
- 2015
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Ingår i: 9th International Conference on Insulated Power Cables, Jicable’15, 21 – 25 June 2015, Versailles – France (2015). ; , s. paper B6.5-
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Konferensbidrag (refereegranskat)abstract
- This work presents studies of the effect of nanofillers on dc electrical conductivity of polymeric materials, potentially applicable in high voltage direct current (HVDC) cable insulation. Samples of low-density polyethylene (LDPE) filled with nanoparticles of aluminium oxide (Al2O3) and magnesium oxide (MgO) up to 3 wt% were prepared, on which charging currents were measured at 40 and 60 °C and electric stress of 32 kV/mm. Experimental results for nanocomposites were compared with those of reference LDPE. It was found that the addition of nanofillers led to a significant decrease in electrical volume conductivity and the decline depended strongly on filler type, its content and surface treatment. Further, the influence of specimen thickness on the conductivity was examined for verifying the operating conduction mechanisms. An indication was found that the space charge limited current (SCLC) mechanism dominated the conduction process in the reference material, while this effect ceased for the nanocomposites.
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| 9. |
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| 10. |
- Kaldéus, Tahani, et al.
(författare)
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All-Aqueous SI-ARGET ATRP from Cellulose Nanofibrils Using Hydrophilic and Hydrophobic Monomers
- 2019
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 20:5, s. 1937-1943
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Tidskriftsartikel (refereegranskat)abstract
- An all-water-based procedure for "controlled" polymer grafting from cellulose nanofibrils is reported. Polymers and copolymers of poly(ethylene glycol) methyl ether methacrylate (POEGMA) and poly(methyl methacrylate) (PMMA) were synthesized by surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) from the cellulose nanofibril (CNF) surface in water. A macroinitiator was electrostatically immobilized to the CNF surface, and its amphiphilic nature enabled polymerizations of both hydrophobic and hydrophilic monomers in water. The electrostatic interactions between the macroinitiator and the CNF surface were studied by quartz crystal microbalance with dissipation energy (QCM-D) and showed the formation of a rigid adsorbed layer, which did not desorb upon washing, corroborating the anticipated electrostatic interactions. Polymerizations were conducted from dispersed modified CNFs as well as from preformed modified CNF aerogels soaked in water. The polymerizations yielded matrix-free composite materials with a CNF content of approximately 1-2 and 3-6 wt % for dispersion-initiated and aerogel-initiated CNFs, respectively.
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| 11. |
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| 12. |
- Larsson, Emma, et al.
(författare)
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Thermo-responsive nanofibrillated cellulose by polyelectrolyte adsorption
- 2013
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Ingår i: European Polymer Journal. - : Elsevier BV. - 0014-3057 .- 1873-1945. ; 49:9, s. 2689-2696
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Tidskriftsartikel (refereegranskat)abstract
- In this study, thermo-responsive nanofibrillated cellulose (NFC) has been produced by the adsorption of thermo-responsive polyelectrolytes to the NFC. Three block copolymers were synthesized in which the polyelectrolyte block was composed of quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) and the thermo-responsive block was composed of poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA). The block copolymers were synthesized employing atom transfer radical polymerization (ATRP) and the PDMAEMA block was utilized as a macroinitiator for the polymerizations of PDEGMA. The length and charge of the PDMAEMA block were kept constant in all three block copolymers, while three different molecular weights of the PDEGMA block was synthesized. The PDMAEMA block was quaternized to introduce positive charges and the block copolymers were subsequently adsorbed onto the negatively charged NFC that was dispersed in water. The lower critical solution temperatures (LCSTs) of the free block copolymers in solution were analyzed by dynamic light scattering (DLS). The composites were analyzed by QCM-D, FT-IR and TGA, which clearly showed an adsorption of the block copolymer onto the NFC. The grafted NFC showed a thermo-responsive behavior in solution upon heating and cooling, thus supporting that the properties of the polyelectrolyte can be transferred to the cellulose. By this methodology, thermo-responsive NFC materials can be produced in a straight-forward manner in water dispersions, without performing any chemical reactions on the NFC.
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| 13. |
- Liu, Dongming, et al.
(författare)
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Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene
- 2016
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Ingår i: Polymer degradation and stability. - : Elsevier. - 0141-3910 .- 1873-2321. ; 125, s. 21-32
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Tidskriftsartikel (refereegranskat)abstract
- Polyethylene composites based on metal oxide nanoparticles are emerging materials for use in the insulation of extruded HVDC cables. The short-term electrical performance of these materials is adequate, but their stability for extended service needs to be assessed. This study is focussed on the capacity of the nanoparticles to adsorb polar species (water, dicumyl peroxide and byproducts from peroxide-vulcanisation, acetophenone and cumyl alcohol) that have an impact on the electrical conductivity of nanocomposites, the oxidative stability by adsorption of phenolic antioxidants on the nanoparticles and the potential transfer of catalytic impurities from the nanoparticles to the polymer. The adsorption of water, dicumyl peroxide, acetophenone, cumyl alcohol and Irganox 1076 (phenolic antioxidant) on pristine and coated (hydrophobic silanes and poly(lauryl methacrylate)) Al2O3, MgO and ZnO particles ranging from 25 nm to 2 gm was assessed. Composites based on low-density polyethylene and the particles mentioned (<= 12 wt.%) were prepared, the degree of adsorption of Irganox 1076 onto the particles was assessed by OIT measurements, and the release of volatile species at elevated temperature was assessed by TG. The concentration of moisture adsorbed on the particles at 25 degrees C increased linearly with both increasing hydroxyl group concentration on the particle surfaces and increasing relative humidity. Dicumyl peroxide showed no adsorption on any of the nanoparticles. Acetophenone and cumyl alcohol showed a linear increase in adsorption with increasing concentration of hydroxyl groups, but the quantities were much smaller than those of water. Irganox 1076 adsorbed only onto the uncoated nanoparticles. Uncoated ZnO nanoparticles that contained ionic species promoted radical formation and a lowering of the OIT. This study showed that carefully coated pure metal oxide nano particles are not likely to adsorb phenolic antioxidants or dicumyl peroxide, but that they have the capacity to adsorb moisture and polar byproducts from peroxide vulcanisation, and that they will not introduce destabilizing ionic species into the polymer matrix. Low contents of dry, equiaxed ZnO and MgO particles strongly retarded the release of volatile species at temperatures above 300 degrees C.
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| 14. |
- Reid, Michael S., et al.
(författare)
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Tunable Adhesion and Interfacial Structure of Layer-by-Layer Assembled Block co-polymer Micelle and Polyelectrolyte Coatings
- 2022
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Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 9:17, s. 2200065-
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Tidskriftsartikel (refereegranskat)abstract
- Understanding and tuning nanoscale structure is critical in developing new coatings and adhesives. In this work layer-by-layer assembly of block co-polymer (BCP) micelles and oppositely charged polyelectrolytes produces structurally unique coatings with wet adhesion comparable to that of mussel adhesive proteins. Cationic (CAT) and anionic (ANI) BCPs, synthesized by atom transfer radical polymerization (ATRP), are used to create colloidally stable, self-assembled, spherical BCP micelles. The assembly of BCP micelle and polyelectrolyte multilayers is monitored in situ where CAT- and ANI-BCP micelles exhibit linear and super-linear growth, respectively. Imaging of the surfaces reveals that CAT-BCP micelles yield flat, uniform layers whereas ANI-BCP micelle assemblies form islands that increase in surface area with each additional layer. The adhesion of these layers, measured by colloidal probe atomic force microscopy (CP-AFM), shows that the distinct layers of CAT-BCP micelle assemblies produce alternating high and low adhesion surfaces whereas ANI-BCP micelle assemblies continually increase in adhesion with each additional bilayer. The unique behavior of each assembly demonstrates that both composition and structure play important roles in wet adhesion of submicron layers and that each can be tuned to target performance for different applications.
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| 15. |
- Sanchez, Carmen Cobo, et al.
(författare)
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Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE
- 2015
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society. - 1944-8244 .- 1944-8252. ; 7:46, s. 25669-25678
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Tidskriftsartikel (refereegranskat)abstract
- Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, a-BiB, an initiator for SI-ATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23?000 and 83?000 g/mol, as confirmed by size-exclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan d values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE.
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| 16. |
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| 17. |
- Wåhlander, Martin, 1982-, et al.
(författare)
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Tailoring Dielectric Properties using Designed Polymer-Grafted ZnO Nanoparticles in Silicone Rubber
- 2017
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Ingår i: Journal of Materials Chemistry A. - : RSC Publishing. - 2050-7488 .- 2050-7496. ; 5, s. 14241-14258
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Tidskriftsartikel (refereegranskat)abstract
- Polymer grafts were used to tailor the interphases between ZnO nanoparticles (NPs) and silicone matrices. The final electrical properties of the nanocomposites were tuned by the grafted interphases, by controlling the inter-particle distance and the NP-morphology. The nanocomposites can be used in electrical applications where control of the resistivity is desired. Hansen's solubility parameters were used to select a semi-compatible polymer for grafting to obtain anisotropic NP morphologies in silicone, and the grafted NPs self-assembled into various morphologies inside the silicone matrices. The morphologies in the semi-compatible nanocomposites could be tuned by steering the graft length of poly(n-butyl methacrylate) via entropic matrix-graft wetting using surface-initiated atom-transfer radical polymerization. Image analysis models were developed to calculate the radius of primary NPs, the fraction of aggregates, the dispersion, and the face-to-face distance of NPs. The dielectric properties of the nanocomposites were related to the morphology and the face-to-face distance of the NPs. The dielectric losses, above 100 Hz, for nanocomposites with grafted NPs were approximately one decade lower than those of pristine NPs. The isotropic nanocomposites increased the resistivity up to 100 times compared to that of neat silicone rubber, due to the trapping of charge carriers by the interphase of dispersed NPs and nanoclusters. On the other hand, the resistivity of anisotropic nanocomposites decreased 10–100 times when the inter-particle distance in continuous agglomerates was close to the hopping distance of charge carriers. The electrical breakdown strength increased for compatible isotropic nanocomposites, and the temperature dependence of the resistivity and the activation energy were ∼50% lower in the nanocomposites with grafted NPs. These flexible dielectric nanocomposites are promising candidates for low-loss high-voltage transmission cable accessories, mobile electronic devices, wearables and sensors.
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