of indoor exposure of plastics isincreasingly requested. For exam-ple, exposing a sample behind glasssimulates sunlight coming through awindow. Using specific fluorescentlamps to generate UVA light simu-lates exposure to indoor fluorescentlighting, such as in a retail displaycase. Solar Light is currently work-ing on accelerating this type ofexposure and correlating it to actual"shelf-time." Thin Film AnalysisPolymer thin films, typically lessthan ~500 nm thick, are used asprotective coatings on windshields,helmets, and other products. Theoptical and mechanical properties ofthese films play an important role inthe overall lifetime of the product,and these properties can change atlower temperatures than they wouldin the bulk material. Cyclical tem-perature changes that oscillate frombelow to above the glass transitiontemperature of the polymer cancause transparent films to become"fuzzy" and flexible films tobecome brittle, for example.Currently, most reliable analysismethods don't have the resolutionnecessary to accurately measure thetrue behavior of polymer thin filmsat the nanoscale.Researchers from the University ofSouthern California (USA) havedeveloped a method to characterizepolymer thin films using hybrid opti-cal microcavity resonators.1Thesesensors operate using evanescentfield whispering gallery mode reso-nant cavities. Under laser illumina-tion, the circulating optical fieldinteracts with the microcavity, andany changes in the material cause adetectable shift in the resonant wave-length of the device. By coating amicrocavity with a polymer film, theresearchers created a sensor thatcould detect the optical propertiesand mechanical behaviors of a poly-mer film. The method is nondestruc-tive, which is important for measur-ing the effect of cyclic temperaturechanges on a polymer thin film. The researchers tested the tech-nique with polystyrene. "We specif-ically chose this polymer because itis very commonly used in foams,food containers, and other commongoods, and it has a moderate glasstransition temperature," says Dr.Andrea M. Armani, who led theresearch team. "We performed themeasurement several times, verify-ing that it is nondestructive, andshowed that the sensitivity tochanges in the optical properties ofthe device was improved by nearlythree orders of magnitude overalternate methods."The technique is ideally suited forthin films and cannot be used onthick samples because the illuminat-ing light interacts with only the first~100 to 150 nm of the film depth.The method is currently useful onlyin a laboratory setting, but theresearchers are working on alter-ations that might make it more use-ful in industrial settings. Developing New MaterialsResearchers at the U.S. Oak RidgeNational Laboratory (ORNL) andthe Technische Universität14| PLASTICS ENGINEERING | OCTOBER 2011| www.4spe.orgA scanning electron microscope image (left) shows the sensor device, which is approximately 25 microns in diameter. An artist's ren-dering (right) shows the laser confined within the sensor device interacting with a polymer thin film. This interaction enables detectionof changes in the film's properties. University of Southern California.
Munchen (Germany) are using theMagnetism Reflectometer atORNL's Spallation Neutron Sourceto analyze new functionalnanocomposite thin films they aredeveloping.2As more materialsincorporate nanoparticles, newtechniques are needed to analyzetheir structures on the nanoscale.The thin films combine Fe3O4nanoparticles with matrices of thediblock copolymer poly(styrene-block-n-butyl methacrylate),P(Sdb-nBMA). The componentsself-assemble into a well-orderedstructure that remains stable withup to 10% nanoparticle content.The challenge in making thesenanocomposite thin films is thatwell-controlled placement ofnanoparticles in the diblockcopolymers can be difficult. Bignanoparticle clusters can form, andsometimes the morphology of thediblock copolymer matrix gradual-ly disappears. The researchers usedasymmetric diblock copolymers,which have two unequal blocklengths. They fabricated and stud-ied composite thin films of asym-metric P(Sdb- BMA) diblockcopolymer and large magnetiteFe3O4nanoparticles (10 nm aver-age diameter). Their samples con-tained 5% and 10% volume frac-tions of nanoparticles. High-contrast neutron scatteringperformed with the MagnetismReflectometer showed that beforeannealing, the internal structure ofthe films was not uniform. Bothwith and without nanoparticles,there was a partial phase separationin the asymmetric films. Afterannealing, the neutron reflectome-try showed that a combined, well-organized two-layer structure hadbecome imprinted in the thin films.Furthermore, the structureremained stable after the incorpora-tion of the nanoparticles. "Thestudies demonstrate that the self-assembly of such materials isstrongly influenced by the energiesgenerated between the interfacingmaterials and the surface energiesof the blocks and substrate," saysValeria Lauter, a member of theresearch team. Analyzing NanomaterialsResearchers from Neaspec GmbH inMartinsried, Germany, recentlydeveloped an infrared near-fieldmicroscopy platform that can ana-lyze nanomaterials with a spatial res-olution of about 10 nm. However,analysis using this platform is usual-ly performed with a tunable infraredwww.4spe.org | OCTOBER 2011 | PLASTICS ENGINEERING |15