Domain: Natural Science — Chemistry
Target concept: Under ultraviolet irradiation, the PLA film gradually becomes brittle, develops cracks, and eventually fragments into small pieces.

Video generation prompt: PLA film was exposed to ultraviolet light, and the morphological changes of the film were dynamically recorded.

Evaluation guide:
(i) Target concept: UV-induced photodegradation of aliphatic polyester (PLA) resulting in embrittlement and fragmentation.
Minimum mechanism: UV photons (UVA/UVB) must be absorbed by PLA chromophores (e.g., trace carbonyls, impurities), generating reactive oxygen species that initiate backbone scission, leading to reduced tensile strength, loss of ductility, and progressive mechanical failure without melting or chemical dissolution.
(ii)Storyline: [Frame 0–2] Intact PLA film under stable UV illumination: uniform surface, sharp edges, no texture disruption. [Frame 2–4] First appearance of sub-millimeter surface crazing: fine, non-branching fissures confined to top layer. [Frame 4–6] Crack propagation: fissures lengthen, branch, and intersect, forming polygonal networks; corners show early delamination. [Frame 6–8] Macro-fracture initiation: edge fragments lift visibly, retaining connection at hinge points; surface cracks deepen into through-thickness fractures. [Frame 8–10] Complete fragmentation: ≥3 discrete pieces fully separated, resting adjacent on background, each retaining jagged fracture geometry and matte surface, all under uninterrupted UV exposure. Perspective: fixed frontal close-up (film fills ≥70% of frame); lighting must ensure surface texture and crack depth are discernible; no occlusion or camera motion.

Manual:
Experimental Procedure
1. Place a flat, transparent PLA film strip on a non-reflective black background.
2. Illuminate it continuously with a visible-purple ultraviolet lamp positioned directly above.
3. Record a 10-second time-lapse video capturing surface evolution.
Observation of Experimental Phenomena
PLA film initially appears smooth and intact; then develops fine surface cracks; cracks widen and interconnect; film edges begin to lift and flake; finally separates into multiple discrete brittle fragments — all while under UV light.
Summary of Experimental Principles
Ultraviolet radiation induces photo-oxidative degradation in PLA, cleaving polymer chains via free radical reactions, reducing molecular weight, and causing embrittlement, microcracking, and eventual mechanical fragmentation.

Prompt Grounding rubric:
level_1: No PLA film is visible anywhere in the shot, no ultraviolet light source is shown, and there is no time-lapse or dynamic recording of any morphological change; the scene instead shows a liquid, a powder, or some unrelated object under no irradiation at all.
level_2: A plastic film is present and stays in place, but no ultraviolet source is identifiable (only a generic lamp with no violet glow, UV housing, or UV label is shown), and the film shows no observable change in surface condition across the whole clip.
level_3: The PLA film is present and is continuously lit by a clearly UV-emitting source (a violet-glowing bulb, a labeled UV lamp housing, or an explicit UV icon); the film stays in the same spot, and at least one degradation cue appears (a crack, a rough patch, or a lifted edge), but it shows up in isolation rather than as part of a progressing sequence.
level_4: The film stays fixed in the frame under continuous, identifiable UV illumination, and at least two degradation stages appear in the correct order (an initially smooth surface, then visible micro-cracks, then macro-cracks or edge fragmentation), all happening to the same piece of film with no cut or swap to a different sample.
level_5: The film stays fixed in the frame under continuous, identifiable UV illumination for the whole clip, and the full five-stage sequence plays out in order: an intact smooth film, fine surface crazing, an interconnected network of cracks, edge lifting or flaking, and at least three separated fragments still visible by the end, with no cut, no swapped sample, and no unexplained jump in the film's position.

Scientific and Causal Correctness rubric:
level_1: The film shows no sign of degrading (it stays smooth and intact), or it changes in a way UV photo-oxidation cannot produce, such as melting, bubbling, turning blue or green, dissolving, swelling, or catching fire.
level_2: Only one degradation cue appears, and it is inconsistent with photo-oxidative chain scission: fragments appear with no prior cracking, cracks appear only on one side while the rest of the film stays pristine with no spreading pattern, or the film mainly turns yellow or translucent rather than cracking or embrittling.
level_3: At least one scientifically valid sign of UV degradation appears (cracking, embrittlement, or fragmentation), but the causal order is wrong or missing: for example fragments appear before any crack is visible, cracks appear simultaneously across the whole surface with no nucleation-and-spreading pattern, or a change happens while the UV source is visibly off.
level_4: All the degradation cues shown are consistent with photo-oxidative chain scission and occur only while the UV source is visibly on: cracks nucleate at a point and spread before any fragment separates, and the film stays solid throughout; no melting, discoloration, flowing, or phase change appears.
level_5: The full mechanism plays out in the correct causal order, only while the UV source stays on: fine surface crazing appears first, cracks nucleate at stress points and branch and interconnect, structural continuity is lost at an edge or corner, and the film mechanically separates into discrete brittle fragments, with no reversal (cracks closing up), no self-healing, and no change that photo-oxidative chain scission cannot explain.

Spatiotemporal Consistency rubric:
level_1: Severe spatiotemporal breaks: the film jumps to a different position or size between shots; the UV lamp switches fully on or off with no fade; the scene lighting jumps abruptly between one frame and the next; the film's shape visibly warps or stretches; a fragment vanishes or appears with no visible cause.
level_2: The film shifts noticeably within the frame between key moments: a clearly visible jump of more than about 10% of the frame width, not a slow drift; the UV lamp switches on or off with a hard cut rather than a fade; cracks appear already fully formed in a single instant rather than growing; a fragment separates with no visible motion, going from attached to fully detached with nothing in between.
level_3: The film drifts only slightly within the frame: less than about 10% of the frame width, a small, gradual shift rather than a jump; the UV lamp's brightness flickers or pulses noticeably; a crack visibly skips a growth stage, for instance jumping from a thin hairline directly to a wide gap with no intermediate widening shown; a fragment's detachment shows no pulling-away motion and instead just disappears from the main film body.
level_4: The film stays visually fixed in place; the UV illumination stays steady and uninterrupted; cracks visibly lengthen and widen gradually rather than jumping between states; each fragment shows a clear pull-away motion (an edge lifting, rotating, or sliding off) rather than teleporting or being clipped out of frame.
level_5: The film stays visually anchored in the same spot and orientation across the whole clip; the UV light keeps a constant violet-tinted glow with no visible flicker; every crack extends along a continuous, visibly widening path; every fragment detaches with a physically plausible translation-plus-rotation motion, stays fully inside the frame, and shows no occlusion or motion blur that breaks the sense of natural inertia.